The foot would be hot and smelly when you pick it up. If worried you can wash the foot with salty water to cleanse it
U will be able to tell and smell, u can see the whole where it has bursted and try to locate where it is and soak it in an anitbacticial wash once bursed to prevent infection 😊
By definition, when an abscess pierces, it opens outwards so you can see pus flowing out.
Except if it is an internal abscess (abdominal, pulmonary ..), in this case you will not be able to observe anything directly.
If the question is how to know if an abscess is ripe, it’s most often a question of time, aspect and feel to touch.
The best is to follow the advice of your veterinarian and / or your farrier if it is a foot abscess.
The Zoetis team