It will smell so terrible you can barely pick your horses feet. Plus it's sticky and needs to be scraped out. The hoof or hooves should be washed out with soap and water.
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.