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There's a bit of trial and error to figure it out, depending on the vegetables you eat. Some things make bad stock (e.g. the inner membranes of a bell pepper or any cruciferous veggies will be bitter), some make little difference (the papery onion skin), others are great (carrot peelings).

A good stock will have some carrots (peels or chunks), celery (leaves are great for this), onion (the ends are perfect), tomatoes (again, the cut off ends are perfect), and probably whatever parsley or any other veggies that are hanging around and will spoil before I eat them. I keep the flavor profile pretty neutral so maybe a bit of pepper and salt but nothing else.

I don't throw in anything that is obviously bad or filthy, but a quick rinse at best is really all you need. The stock will be boiled at least once, more than likely twice (when you cook with it) so I'm more concerned with flavor than bacteria.