If you mean Thrush in the hooves then it's quite an obvious one. It will smell vile even if you've picked out the hoof & there will be a black, slimy looking layer in between the frog & the sole. It will scrape away with the hoof pick. The horse may be footy too, if barefoot.
A good treatment is a diluted antibacterial cleaner. I use diluted Detol & scrub the feet 2/3 times a week.