For starters, absolutely do not trust any websites blindly. They tend to put a lot of (popular) music in the "wrong key" either to make it easier to play (conunterintuitively, mostly when it's a harmonically simple song), or just because they messed up. This problem ravages guitar tablature sites in particular.
To solve this, I recommend listening to whatever recording your source material is (e.g. Bing Crosby's "White Christmas" recording from 1942) and determining the key directly from that audio. Other options include simply checking a bunch of websites for independent verification (which could still end up being wrong) or learning to play the song in all 12 keys. I find that checking piano sites tends to be more fruitful than guitar sites, for multiple reasons that I'll not explain here. (the worst kind of site has to be ukulele tab sites; they post songs in approximately three of the twelve keys, nearly everything else is transposed)
Note that for some songs, there really isn't any correct key. Take Happy Birthday, for example. What key is that in? (Bonus points if you say "all of them simultaneously") Or "The Star Spangled Banner"? Or the "shave and a haircut" bit?
And if your band or you wants to play in a certain key for the song, then do that, whatever the reason may be. Transposition time.