This is interesting because I work at a clinic where vasectomies are done, and I've seen the other providers just look at semen on a slide. Our scopes are ok, I wouldn't say great. This is a clinic that typically just provides GYN care, so what we do with our scopes is pretty limited. As I said before, I've seen semen in vaginal secretions and it is pretty obvious. Maybe it's harder if they're non-motile? I'm assuming dnw826, that you were looking at sperm morphology and not just presence of sperm, so you probably had a kick-a$$ scope.
A quick peek under a scope would not be as accurate for determining any sperm presence without several samples taken, and a good scope. I would be wary of anyone telling me we were in the clear after just looking at one sample with a normal scope. I would request another sample and a good powered one. I might also suggest them spinning it down to get rid of the superfluous fluid to try and see if there are stragglers in the bottom.