I removed one today (need to do a further check but gelding had enough today as had started with just general sheath cleaning, so left it while he was still tolerant). I had my suspicions that he had one (nothing major, just occasionally taking his time to urinate). It was quite soft and came out fairly easily once I found it. Haven't ridden since its removal though so can't say if he's any different.
I get the vet to sedate and clean yearly. I try to give him a basic clean regularly but he gets a bit 'stampy'. He is a very big horse so it would be far too easy for me to miss something when battling with an unhappy horse.
I was told many years ago that when you hear that creaky noise (that you'd almost miss as a leathery, tack related noise) in trot, that meant they needed a good clean. No idea if this is true or not.