bandettes or zip ties on the legs. You can also use a crayola washable marker, depending on the chick color, this is very short term though and wears off.
You can toe punch most poultry. I couldn't find a way to mark chicks that were day olds. I tried food coloring, markers, etc.... I finally trimed the wing feathers on one side. Every few day I would check and trim some more. The feathers grow sooo fast. When they were about 6 weeks old, I used leg bands.
I couldn't use a toe punch. Not enough webbing on the toes. How do you guys get the zip ties off? Their legs are so small. Nail clippers, scissors???