[Kneel B4 Zod]

Sub shops:

Meatball subs. At once, this requires only 'simple' ingredients, but it's pretty easy to tell if they are using high quality ingredients or not that will be found in other stuff.

Meatballs - should be easy to tell how good these are. nice homemade meatballs, or grammar school cafeteria style?
Sauce - ditto. pretty easy to tell the difference between bad and good sauce.
Cheese - kind of tricky here, b/c when it's melted and mixed in with the above 2 it can be hard to pick up. but I've had off tasting provolone before = very bad
Bread - fresh and baked just right?

so 4 basic ingredients (although good meatballs and sauce require many fresh sub ingredients) that should give you a good indication of how good a sub shop is. a surprising number of places can't make a good meatball sub.