Stinky feet (it has a pretty bad smell) is your first clue. It is kind of hard to miss unless it is hidden in folds (in which case, the folds need to be examined and what can safely be removed - removed, and the the rest treated). Otherwise, as a rule the top layers of the infected area are discolored (may be hard to see on a black frog) and "soft" ( can easily
be scraped off). If my horses have stinky feet, but I can't clearly see thrush, I treat them for it anyway w lysol, betadine and 50/50 ointment - none of which will hurt anything.