Great response from Aljeres---that's how I do it---wait for the "resting tone" or "resolving" note at the end of a verse or phrase. It was hard for me at first because I play for a jazz vocalist who knows her keys!! I play the intro, she starts singing--NO STRESS..lol
I've often said that I wish church singers knew their keys---when they come up, why can't they tell the musician "Pass Me Not please in Eb??" That's not so hard to do. Carry a songlist of tunes & the keys in their wallet or purse? When I play for guests soloist, after church I tell them the key they were in and say: In the future, you will know what to tell the musician!
Be Blessed!
Lisa