You can smell it and if it is bad it will be black and oozing.
Some will not agree but my vet told me to use hydrogen peroxide (from Boots) squirt it into all the crevices with a syringe then spray with tetcin (sp) blue spray.
I had been fighting with it for weeks and getting nowhere with sheep foot rot spray amongst other things (honey!)the peroxide sorted it in a couple of days. You just have to be careful as it does kill healthy tissue too.
Hydrogen peroxide is good - I tend to squirt and then scrub a bit in the crevices with an old toothbrush, then rinse again. You can dilute the HP with water so it is not so active (get 3% by volume and you should be ok anyway).
Also sterilising fluid or tablets dissolved in water are good for a weekly slosh over and scrub to keep things healthy when the thrush is gone - don't go mad though.
if your horse prone to thrush make sure your farrier trims frogs back and tidy. keep on a clean bed, and out of anything soggy!!. peroxide works well, as does stockholm tar and sheep foot rot spray!!
As others say, smelly feet and the frog looking like its being eaten away! are the signs, try to keep feet dry, i.e clean dry bedding when stabled and provide a dry area to stand on in field if out 24/7. Treat with hydrogen peroxide to kill it off then once dry I use terramycin (blue) spray.
I have also found my mares feet to be worse when she's out on a field previously grazed by sheep, apparently the urine of sheep can react with some horses feet and cause thrush.
Where do you get blue spray from? I'm currently battling with thrush as well. I found that Hoof disinfectant spray (Equimins) is quite good, and am currently using Nettex Hoof Master to get to the heart of the matter. We seem able to control it, but not cure it.
I've been fighting thrush in my oldie for years. I finally seem to have cracked it. Strangely enough at the agricultural chemists.
I was spending a fortune on foot rot sprays and other stuff even though it was cheaper from them. Got chatting one day and he suggested some hoof disinfectant that they sell for sheep.
Its sold in gallon tubs and used neat on sheep as they only catch them occasionally, really nasty stuff. But it can be diluted and put in a spray bottle.
I decided to give it a go and haven't looked back. I dilute it more than they recommend and just spray it on when I clean his feet out. I dont think I could stop using it but his feet are more or less normal now.
I will find out what its called when I go to the stables later.
It cost about £35 but you could do a whole yard for 5 years with the contents
Quick addition to the above - iodine, diluted approx 50/50 is a very good cure. As stated, peroxide is also good and getting your farrier to keep frogs trimmed and tidy helps too.