Talk to members of other guilds - obviously be polite :-D Try the fleet as there are usually plenty of people around there. Ask them about membership numbers (ACTIVE not actual as there can be a large discrepancy between the two) and get a feel about what the ethos of the guild is. TBH that can only get you so far as all guilds I have joined, in previous mmos are in a constant state of flux - social ----> hardcore raid ----->casual raid --------> social, etc (and any recombinant version of that) meaning that a guild I joined because (at the time) we suited each other could, a few months later, be totally wrong. You won't get grief for picking and choosing if you find it takes a few tries before you feel at home - just let the leaders know it isn't personal (unless it is and then I want to see the flaming screenshots posted here!!) and move on.
Hope this helps