There is a lot of work that traces developments in Plato's ideas, and a rough consensus that some dialogues are more Socratic and others reflect Plato more (http://plato.stanford.edu/entries/plato/#HisSocEarMidLatDia). There's a mixture of stylometrics, contemporary accounts, and other arguments that are used to try and assign responsibility. It's an inherently imprecise project, and I didn't do much ancient philosophy so I can't tell you much about it, but it seemed sensible when I took courses on Plato.