How to check copyright?

I went through this many moons ago when one of the start-ups I worked at was working on an emulation layer. The lawyer made a distinction between "retained knowledge" (i.e. what was in our heads) and copying verbatim from either the files or notes. I had to hand in all my notebooks when I left the company but assuming no reference I could implement something the roughly the same way I had before. There is a lot of code which isn't copyrightable because it is either the only way to it or its "obvious".

Patents where a separate legal rabbit hole.