Ever try to grade 30 or 40 homework papers night after night? I grade primarily for completion in math, but the students know that I consider it their responsibility to bring up at the beginning of class on the date due any problems they had trouble with. If they don't, and they blow the next test, tough noogies.
I meant they just look at your homework paper and see if it looks complete. If it does, you automatically get full credit. They don't even collect it. Once one of my math teachers asked the kids to hold up their worksheets they did for HW. Since she had no way of specifically seeing them, one kid in the back who hadn't done it held up an English paper and got full credit.