Help on some sort of riddle?..?

Imagine that you were a caveman with a herd of sheep and in the morning you would take the herd you have out of their enclosure and take them up to the river or something. then you rest bla bla then go back. how would you know if you came back with the same number of sheep?.. this was before counting was invented and you cant use pebbles or anything of some sort to keep track of them or watever. ? helppp?