There's two colleges near me I do not attend. I've just walked right up to professors, talked to them for a few hours fine. I've even been engaged in research projects, while both not their student, and not attend the college. In many ways, this question is a lot like asking how you get anyones attention; productive talking is effective.
There are things to keep in mind of course:
- Talking to random professors or about random subjects is not going to go over particularly well. One personal example I have is I deliberately sought out a professor of mathematics, to discuss something mathematical; keep it relevant.
- Trying to get their attention at bad times is not going to work; learn their schedule and find a time that works. Even better if you can set up an appointment (emails works, in-person has a nice touch).
- Don't demand. You have no idea how often I've seen this, even from students. It doesn't go over well. Ever!
Good luck!