For myco it's a simple blood test, costs just over £20 here (Herts) though one vet wanted about £100 for it
The test covers myco G, myco S and Marek's disease.
Tylan has worked well for treating myco G, but yes, they'll still infect the others. Having said that, mine hardly ever show symptoms because they're hardly ever stressed. I just know this group carries it because a Marek's test was done a few years ago. They've lived with it quite alright, really.