You can usually tell an abscess because the horse will have a low-grade fever and the hoof will be warm. If you put a hand around each hoof very early in the morning , you can feel that the sore one is slightly warmer. Early in the morning, all four feet should be very cold to the touch. Once a horse starts moving around, they all warm up and it is much more difficult to tell.
We started letting all abscesses come out at the coronary band many years ago. That is what is natural for them to do. Years ago, we let farriers and/or Vets dig them out and had several that took months to fill back in and required pad which encouraged soft feet and thrush.
When we moved here, we had several horses abcess because we changed from a very high dry desert environment to a wet one with more rain, mud and ponds. All of our horse's feet got soft and bruised and abscessed. The first Vet and farrier we used started digging them out. We ended up with horses that were unridable for months and the Vet crippled one. Then, we changed Vets and started using the horseshoeing school in Ardmore, we were taught to let them do the natural thing -- break and drain on the coronary band as long as there was not a foreign object in them.
As long as there is nothing that has penetrated the hoof and is still in there, they will get very lame, break and drain on the coronet, dry up and go away. We do not even doctor them any more. We just put them in a field that has no slimy pond to stand in.
If an abscess in a hoof has 'puss' in it, it probably has a foreign object in it -- or did have one. Most hoof abscesses have a black watery liquid in them. After they break on the Coronary band, a little 'blister' grows down on the hoof wall. It grows out and is trimmed off without any consequence as all.
Since we changed tactics, the Vets and the horse-shoers make a lot less money and the horses are completely better much more quickly.