A good way of checking what key is most comfortable for you is to have a chromatic tuner handy - then when you catch yourself singing off guard, you check what the final note in the song is, and with most songs, that'll be the name of the key (mind, it still could be major or minor or in some mode or other.)

One thing that can confuse things is when someone asks for a note to help them start, and the people playing the music assume thats the name of the key the song is going to be sung in, and there's no reason it should be, if what they are after is the first note of the song.