many of beattles' songs were co-composed by john lennon and paul mccarthy. in recent years paul had indicated some of them were mainly composed by him but of course mrs lennon didn't agree.
paul and john did have different styles and it would seem to me easy to tell which co-composed song was mainly written by whom.
if you can tell 李白 from 杜甫, or mozart from beethoven, you can tell paul from john.