Cattle here are verified by dentition.  For now. 

Direct quote from the CFIA handbook for export inspection.

"For the purposes of export certification of cattle and bison to the USA, an animal will be considered to be 30 months or older if either tooth of the second set of incisors has begun to erupt through the gums."

All cattle to be exported are required to have a veterinarian physically inspect their mouths. 

This is the same standard used at the slaughter plants.