Symptoms may include limping or favoring of a leg. Bumblefoot is a staph infection of the pad of the foot that is often blocked by a hard plug, you may either take them to a vet for treatment or treat it yourself by removing the plug (often with a scalpel or tweezers), flushing the inside of the foot thoroughly, and wrapping with antibiotic ointment such as Neosporin. Some birds will have chronic bumblefoot, which will recur repeatedly.
Best way to avoid it is keeping a clean area for them free of thorns or sharp garbage that may cause infection of they poke a hole in their foot.