Heartworm and symptoms

My dogs been on heartworm meds for over a year. He isn't due for his blood test but I was wondering what the signs are if he has heartworms and is on the meds at the same time.

[Jaybay]

If he's been on heartworm preventive regularly, he shouldn't have heartworms.  Is there a particular reason you suspect he does?  Often there won't be any symptoms at all, which is why vets do an annual blood test.