The first question you need to ask yourself is: would disclosing this help you? If your list of accomplishments in the position looks lean for the amount of time you were there, but that's because you only worked there half-time or less, then it's potentially worth indicating that it was a part-time job. If your resume wouldn't raise eyebrows if you didn't say this, then don't. Remember: the purpose of the resume is to get the interview; later on you will disclose your job history in much more detail (possibly including positions you omitted entirely from the resume), either verbally at the interview or via a written application. If the company doesn't seem to be the sort that has formal written applications, then I suggest bringing it up in the interview so they won't later feel you've misrepresented yourself.
If you do include it on your resume, readers will naturally wonder "part-time? what else was he doing?". So make sure your resume (or, if necessary, cover letter) has an explanation for that, or you might find people passing on your resume before they ever talk with you. Job dates that overlap education dates are a pretty clear signal.
As for how to indicate it, "(part-time)" after the job title is the usual way in my experience (US, high-tech sector).