RE: How do you know that...
Common sense really will ultimately let you know.
Look for the presence of sign.. ie rubs etc.
But the number 1 thing I use is my sense of smell. Older whitetail leave BIG beds.. and when you smell them and you're in their bedroom.. they stink. A musk thats easy to recognize.. and seemingly only carried by the older bucks with established (if you could call it this) bedrooms.
I don't know if you do any shed hunting.. but if you do.. the next one you pick up thats fresh from that spring.. smell it.
Their beds smell like those sheds.. only muskier. Once its in your nose.. it's there forever.