As others have already said, it's a bit of a personal issue... I would say if you're not sure about a game, do a few things.
1) Ask someone at the local store about it, even to demo it if possible. My local EB is great for info on games (though not usually from the same perspective as what you're asking about it).
2) Read on-line reviews. In addtion to secular reviews and such, there are seveal Christian sites out there that do game reviews (not that I agree with their view most of the time, one site reviewed Halo and said that it "promoted violence" a term I think is way over-thrown around in Christian circles these days).
3) Rental time. Blockbuster and Holywood video both rent games, many of the newer ones should be there (though you may have problems finding a copy of newer or popular titles, go on an "off" day, such as Tuesday, you know, the middle of the week). You've only thrown away $6 at most if you do this, even less if you look for the mailers from those companies that give you 99cent rentals and such.
oh... and i guess
4) ask around online... like here, but when you do this you'll want to have an idea what is "out of bounds" for you.... what you don't want to see... just asking if it's "bad" is usually a loaded question (I'm a fan of the Harry Potter novels, a split issue in Christian circles and as you may see here, people here play some scary and violent videogames).
Hope my pointers are of some help...
PHIL