If you're in college, look around the college clubs - most clubs will charge around $50-80 a semester, while other private dojos/studios will easily charge that per month.
-If the instructor tries to coerce you into joining, leave. You're in charge until you sign up.
-Try to ask as much questions as you can. I know it's kinda intimidating to do that in person, so you might consider emailing him/her/it first: it'd be a good idea to contact them before you arrive too. Common courtesy.
-Most places will allow first timers to try out for the first class. If you don't like it, leave.
-Try to get a feeling of the people in there. If you sign up, you're about to spend a lot of time in that place, and usually people of similar personalities gather at different schools. If you don't like the instructor/student, it's probably not a good idea to stay there.
-Visit as many places as you can. Let them know that you're looking around, and give them a timeframe by when you'll have a decision made. stick by it. If you find a studio you absolutely love, you might find the next one even better.
-If you know anyone who's been in this stuff for a while, ask them to accompany you as well.
Good luck!
And just out of curiosity: why JKD? I'm kinda biased against Bruce Lee, I suppose, but why?