Firstly, clean the foot (soak in Epsom salts + warm water / betadine + warm water) and wipe off all dirt with a paper towel. This allows you to clearly see anything on the foot. If it is a black scab, it could possibly be bumblefoot. Keep clean, and apply an antiseptic spray for birds, if it is a mild case, this could be enough to overcome it. Look for signs of limping, hesitation to walk or swelling. If it continues to swell, chances are that it is bumblefoot, and a more serious approach is required. (Surgery)
Ensure the coop is kept clean, and the perches are splinter-free, as cuts on the feet, and dirty housing causes bumblefoot.