I wish we had the space to keep a couple .
In England we have Donkeys that live in the New Forest in Hampshire , free ranging.
They do have owners but are keep in herds on the woodlands and forest .
We also have beach Donkeys , they are very well protected by law .
They only allow to work a few hours a day ,for a few days a week and must have a cool shelter, feed and water.
An inspector comes everyday before the donkeys give rides, to see if all are in first class condition and the owner has fulfilled all the legal requirements .
They spend the rest of the year on their holidays at stables .
Only Christmas and Easter time they come out to take part in the Nativity and the religious Easter walk.
I believe, like horses, you look at the wear on their teeth. Can't cut them in half and count the rings. If they have a tattoo in their lower lip in front, sometimes that can get you info too. This is horses do not sure about donkeys. Other thing you could check for is a microchip. You'd need a scanner or a vet with a scanner. Could give you an approximate idea anyway.