You don't want to be 'that guy' who picks the team that isn't at the event (happened today at West MI). What we do is have a team of 6 people watch the robots, and enter them into
our database. On Friday night, we all get together (all 20 people who are interested), and make a preliminary picklist, including among other things, a "don't pick" list. We also figure out what our alliance should be made up of (ie, triple balance alliance; picking for hybrid points). After that, on Saturday, we finalize that list, and go around to every robot on it, and make sure that they are fully functional (we've been burned on this before).
However, if you don't have the resources for that, I would suggest watching lots of matches to get a feel of the field, and using
Cowscout for the picklist. It should get you a pretty good one, though not perfect if you're trying to create a specialty alliance.