Hi Dave. I didn't want to come off as a know-it-all; I don't.
The most obvious difference is the B-50 has a distantly higher fin and rudder. If you look at the engine nacelles, all 4 of the B-50s extend past the trailing edge of the wing; only the inner 2 of the B-29s extend that far. The B-50s have more external aerials. Many B-50s have fuel tanks mounted under the outer wing panels.