Hey, Billy, of the limerick.
You are right about stage vs. film, but I'm afraid that I am no expert in the legitimate theatre.
As to being able to tell when one actor is better than another, it's a matter of feel for me. If I can identify with the role, then I dub that actor a good actor. Kevin Spacey very rarely changes expression in his performance, but there is something about him that absolutely says:
" You are one helluvan actor, Kevin."
To date, the only movie that I have seen in which Spacey was NOT good, was "Pay it Forward".
There are other actors, of course, which I deem good, but at the moment, I can only think of one, and that is Spacey along with Kevin Anderson. (think that's his name). He never looks the same twice. He was Bobby Kennedy in "Hoffa".