Maryrrf
You remember the key you sing them in.

I don't know the key's of the songs I sing.   I start. If it's good I carry on.   If not, I know I will not be able to sing some of the notes.   So I stop and start again (explaining to the audience that I haven'e got my tight underpants on).

As for crib sheets, it depends on where you are singing.   If you go somewhere regularly you can't keep singing the same songs every week, so you sometimes have to use a crib sheet to remember new songs.   (A good way to do this is to have someone else reading the sheets, if you get stuck thay tell you the next line.)

The silliest song I forgot was the January Man when I got the mounths mixed up.

Paulo