Sometimes I will check over there math paper and mark the ones that are incorrect. I also try to create a culture where students don’t admit they are finished with their homework. I keep a stack of fun but hard math/logic puzzles around that they must work on when all their homework is done. The alternative to doing this is looking at my books/national geographic magazines that are available. If student asks to do something because the are finished I usually respond with “Find something quiet to do (while pointing to my book shelf) or I will find something for you to do.” After a while some students don’t ask for things to do, others never get the hint.