for this there are sperm count tests carried out at your local vet practice (they cost little money and, more importantly, without risking of having a litter of malformed pups or jeopardise the reproductive health of the bitch). for the sire (or potential sire) trying him at stud when not ready can cause serious behavioural issues and the fact that once this method has worked does not mean that that's going to happen in any case.