How do I tell if a used bike (craigslist) is worth it?
I would say that most people on Craigslist would be willing to let you give it a test ride, I mean it's similar to buying a car. Just, as I always tell people, meet with the person in a very public space. Much safer place.

As for what to look for, I would do a little research into exactly what you are expecting to use the bike for. Figure out the kind of bike you need and do a little research on the kind of mech recommended for what you're doing,(frame, gears, wheels, shifters, brakes, drive train.) Once you figure all that out, you can narrow down your selection on Craigslist. Just like looking at different models in the store, online you will find pictures, descriptions, e.t.c. So you can find the right bike for you.

Once you think you've found one, speak with the seller and find out more about the bike. Research things like the bikes brand, how old it is, e.t.c. Once everything checks out, boom. You have your bike!