How do you distinguish the proto-lithology of a metamorphic rock?

Its really hard to describe the issues in hand samples and thin sections.
Stuff like the rising temp mineral assemblages are unique to many metasedimentary rocks, while crystal shapes and mineral assemblages are unique to met igneous rocks .
You need to get a good tech book on petrology and look at the information on field samples.

If you have access to thin sections of samples (at cross section and planar), as they say, a picture worth thousands of words.

Is this a class or a hobby?.

Why not start with something you know the geology of, like schiists or gneisses and look at the "country rock", then look at the structure and mineral assemblages (degree of metamorphism imprts certain minerals behind, like garnet or sillimanite)

Also the style of metamorphism, imprints a unique assemblage and structure on the rocks .

thanks that actually helps a lot! I'm currently studying for a petrology class.