This should be a small, subtle feature. One horizontal dimple or crease is usually ideal. The cuff should indeed rest on the top of your shoe — there needs to be contact — but it shouldn’t do much more than that. The trouser can fall a touch longer in the back than in front, so long as it’s still above the heel of the shoe (the actual heel, not just the back of the shoe).
This is one of the easiest adjustments to make, so you can rely on making some changes here if you need to. In fact, dress pants are often sold unhemmed, with the assumption that the purchaser will take the trousers to a tailor (or make use of the store’s tailor if there is one) to have the cuffs fitted.
A good fit in the seat will lie loosely against your underwear, without pulling tight against your butt or draping loosely down your thighs.
You can spot a bad fit in the seat when there are horizontal wrinkles just under the buttocks (caused by too tight of a fit), or by loose, U-shaped sags on the backs of the thighs (caused by too loose of a fit).
A tailor can “take in” a seat to make it tighter in the back without too much difficulty, but there’s a limit to how far he can go. If the seat was way too loose to begin with, it’s not possible to adjust it to fit without pulling the pockets out of place.
Unless the pants have an unusual amount of spare cloth on the inside, seats can’t be “let out” very far to make the fit looser. Err on the side of too loose rather than too tight when buying.