Firstly, I know you probably do not want to hear this, but it is important to bear in mind there is no 'for sure' way to tell how well a phone interview went, regardless of your immediate impression, whether it be 'bad' or 'good'.
It is hard enough trying to tell how well you did in a face-to-face interview, but here you are at even greater disadvantage due to the absence of seeing and feeding off the interviewer's body language etc. Furthermore, bear in mind, like all interviews you are likely to come across differently in your interview to different people i.e holding all else equal, the interview will have gone well to some interviewers but went poorly to another interviewer. Hence, although the following points may be on the 'average' indicators and signs that point to an interview that went good (or bad), that does not mean ultimately you will get the position, whether that be for a college interview, internship interview or job interview etc.
Moreover, you don't know how well your competition did, you might have done well, but they might have done even better. (Here, however is how you can beat even the best competition in the phone interview round, with the "The Essential Phone Interview Handbook - it is a unique and powerful tool that will teach you the necessary phone skills to impress any potential employer and ace your way into the job you want -- Ron Fry who is the author of splendid best-selling book 101 Great Answers to the Toughest Interview Questions - The Essential Phone Interview Handbook contains great advice and guidance right from the psychology of the 'questions and answers' to the phone interview practicalities. Also be sure to check out 'the preparation plan' for an upcoming actuary phone interview -- where you can learn the type of preparation that should be involved to really succeed in a telephone interview ).
How to Tell if Your Telephone Interview Went Well? - The Signs & Hints
The Questions Asked
As it is a phone interview, you obviously can not gauge their body language only their tone and the styling of the questions they provide you. Hence, it is logical to take encouragement from an interview who is firstly polite to you and is actively engaged with the answers you are providing (hence listening to you attentively). You should not feel like the interviewer was trying to catch you out or making you feel dismissive and disinterested in your responses. Instead, there was a sense of agreement with your answers an air of concurrence. Was there a rapport built between you and the interviewer, put loosely, did you feel you could have carried on your chat over a drink and a reasonably good time with it (all professionally of course)?
An Obvious Word of Warning: Interviewers have different styles and characters (some are generally always polite - therefore hard to gauge, and some are more abrupt - hence you may have felt a false disinterest in you) therefore I would advise to not read in to this too much as a sign as to the interview going well or bad.
You Bring More and Then Some
There is a point in the interview where it is quite clear in your answers that you have what it takes to do the minimum requirement for what they are looking for. However, you have also identified areas within your experience and skill set where you can be of additional help, where you can add 'extra value' to this company/institution. A skill set for example where you bring that little bit 'extra' going beyond what the job position is asking of you, and you start to revolve around those particular aspects, giving them an extra skill set they did not expect or know they needed. In which the telephone interview does delve into certain aspects of this skill, and you should take encouragement from questions being directly asked upon this 'extra skill' this 'value added' you can bring to the company. Already making yourself partly indispensable and certainly differentiate you from the rest of the pack.
What kind of skills could these be? E.g. Advanced Computer Skills - Where you meet the basic computer requirements and surpass them, by not only knowing how to operate Excel, but you know computer code etc.
The Longer... the better.
Obviously this is sign is a rather 'big' generalisation, however, you should take comfort in having a long interview, where you have both gone over the allocation of time provided for the interview. It makes logical business sense, from your point of view as the interviewee. It shows the obvious indication they are 'interested' in you. A potential employer will not waste time i.e 'interviewing time' on a candidate they do not feel that is right for them. Logical business practice.
Furthermore, generally in a case where the interviewer has run out of the 'procedural questions' and instead has led on to more in depth questions about your experience and what you can bring to the job and their institution or company. These type of questions are usually associated to job responsibility roles, they are kind of just simply checking of the checklist with you in a manner of speaking, such as "Will you and how would you be able to deal with (X) responsibility?". Again, a positive indicator as to the interview going well.
Further Engagement 'They appear happier, than you are'
A grave although not fool-proof indication as to whether the interview went well is a 'second interview' (obviously if the phone interview is another meeting with you, should take comfort in knowing they are already interested) or a 'we will get in touch with you soon to organise further details' with a specific time frame, not one which is generalised. Any proposed further action on your behalf is a positive sign, if they are not interested in you they will generally ask you not to do a lot (not always the case mind).
Moreover, if they ask for references and their details after addressing you and your resume (CV) or in actual job details such as what are you expecting from them such as salary expectations and roles. Any indication really that the interviewer seems to be wanting to know past your characteristics and personality (you have already passed that test by now) and more specifically towards this new role you are applying for, should be noted as good sign.
Perhaps a development where you already have started to 'plot' what you are going to do as soon as you start your first day. Furthermore, if there were any subtle indications as to the interviewer selling you the role or if they mention other candidates in a dismissive way or you in a level above the rest, again should be noted as a positive signal for you.
Although the indicators above are 'positive' and some 'negative' signs it is worth repeating you can never know for sure. In truth there is only one real sign that your interview went well, and that is if you are asked "When can you start?".
There will of course be interviews where you think they could not have gone better but ended up rejected and of course you may have thought you could not have had a worse interview yet some how ended up with the job or position. But ultimately and probably most importantly, do not stress over how the phone interview went, asking your self "How do I know if the interview went bad or good?" should be for the sole purpose of simply asking on how you can improve your ability as a better phone interviewee rather than relating it to the success of getting through that round or job etc (but of course it is entirely natural).
Once the process has been carried out and find yourself unfortunately find you are unsuccessful be sure to ask for constructive feedback, this will obviously help you on your way on areas to improve.. But, remember what's done is done, you can only do better the next time. I hope that this article has been yet of some help in letting you know at least to some degree hopefully in how to tell if a phone interview went well.