How were cattle marked for ownership?

With ear tags. That and by looking at individual cows. Some producers are really good by being able to tell, just by looking at an animal which cow is which without having to look at the number on their ear or brisket tags. If a producer has a herd of cows that vary in colouration, then it's easy for him/her to tell which cow is which. It's different, though, if they have a herd that is uniformly black or red. Then they must make use of those ear or brisket tags for better identification.