It's hard to know until you do hang out with someone. Until you do get to know your coworkers better, just treat them all the same: with respect. The first rule of socializing at work (and the first rule of work in general) is to treat everyone with respect, from the receptionist and cleaning crew to your colleagues and the firm owners. No matter what you end up doing or with you end up being friends, treating everyone with decency is the best way to go. Even if you don't particularly like someone, you can still get work done with no hard feelings.
That being said, most folks generally know how to make friends--be nice, say hi, talk about stuff, hang out, etc. The thing about work friends is that you have to ensure that your friendship does not affect the quality or quantity of work that gets done. While this first rule of socializing at work is to treat everyone decently, the first rule of work is to get your assigned work done, and do it well. Some people will always do a good job, no matter what. But some people will shirk their duties and want you to lie or fudge or cover for them. Some people will complain about situations at work or gossip a lot and want you to engage in those behaviors with them. If you're not comfortable with doing these things, then it's imperative that you speak up and set your boundaries, which can be hard to do with your buddies. Some people will believe that you have to put up with cruddy or unethical behavior out of them because you're "friends", which is why it's doubly important to learn as much as you can about your coworkers before you hang out together outside of work too many times. If you can avoid the whiners and slackers and cheaters before you ever become good buddies with them, then you draw your boundaries with them even more clearly. There's no "if you were my friend, you'd say I was in at 8 when I really came in at 10"--they know they can't even ask you for that favor in the first place.
Even if you have good friends that you work with, and you end up hanging out a lot outside of work, I believe it's still being a good friend to be the voice of reason/neutrality sometimes with your work friends. Occasionally, my work friends (and my husband, with whom I worked for over six years) would hear me complaining about something or someone, and they would either agree with the other person or at least play devil's advocate with me. They didn't automatically side with me just because s/he was my friend or because s/he couldn't stand So-and-So either--they would be tactful but honest. I'd like to think all of my friends would do such a thing for me, whether or not I worked with them.
If you have a question you'd like to have answered or a topic you'd like to see covered in a future post, please post it in the comments or send me an email from the sidebar. Thanks!