You're right when you say it's sometimes hard to tell when someone slips into a couple of notes in falsetto. Some people glide in so smoothly, I can't hear the change. If someone just slips in occasionally for a note or two, it might well pass unnoticed. In black gospel, the singer is likely to do whole lines, or the whole song in falsetto. It's much more obvious, then..
Falsetto in country music or bluegrass would make more sense, as yodeling was taken in as part of western music. Guess there's another thread there... one on yodeling..