The only ways to check for the adult worms are with an echocardiogram, or with an MRI of the heart. If money is no object than having an echocardiogram performed may set your mind at ease. An MRI would cost over $1000.00 each.
I think it is less of a gamble to treat them for heartworm disease than to leave the heartworms there to damage their hearts.
Anaphylaxis reaction from the dying worms is a possibility, but I am sure your vet will do everything possible to prevent or to treat such a reaction.
Thank you for your advice.
Both of my dogs are now having their treatment. So far so good.