When a calf is born we put an eartag in her ear and record it. We also photograph the calf in case she ever loses that tag. We've been able to identify many over the years this way. When she calves we give the cow a chain with a number on it, like a graduation into the milking herd present. Sort of like a Hawaiian lei done dairy style.
Now in a perfect world I would have some kind of gps tracking device on our cows and calves so I could easily find their exact location by looking at my phone. "Look! Old 237 has gotten out of her field again." I think that would be pretty awesome. Maybe someday.