It also depends on the type and size of dog, and the quantity of the "can't". A little bit of ham won't hurt them, nor will mashed potatoes or stuffing. But garlic, leeks, onions, grapes, raisins, and chocolate, are major no-nos. Raw dough can rise in their stomachs. But a large-ish dog with no digestive issues can "clean" a mostly empty plate and be fine.
my dog chewed up an ant trap once and lived 13 more years, I concluded she can eat anything but yes, best to check first. I'm always going, "hey Google, can dogs have _____?"