Daant -- the number of permanent incisors -- is how age is judged at the mandi, and age is not optional: a goat or dumba must be at least a year old for Qurbani, a cow or bull two years, a camel five.
The problem is that sellers know exactly how much the answer is worth, and age fraud runs in both directions. A young goat is sold as older to be valid, and an old goat is sold as younger to look better value.
So: how do you actually open the mouth and read it? What do you look at besides the teeth -- horns, hooves, body shape, how the animal moves?
And the wider question, since teeth are only half of it. How do you check for health on the spot? The usual list is a shiny coat, clear eyes, alert behaviour, no limping, no wounds, and no discharge from the nose or mouth, but there must be more that experienced buyers look at first.
Post what you check, in the order you check it.