I'd be hesitant to trust a shop worker at a feed store unless they have a ton of poultry knowledge (or somehow they're a vet :)). I freaked a little bit with one of my roosters this summer when it was warm, but not hot. He kept panting and that was the only "symptom." Now that the weather is very cool and night and during the day, he doesn't pant at all.
Folks on here, rightfully said to relax...and to let him be if he had no other symptoms of illness.
If the bird has bright eyes, a good appetite, fine poops, and is acting normally, I wouldn't treat them for anything if all they do is yawn. When the birds have a respiratory infection (I've only had one bird with one that didn't make it - he was sick when I got him and eventually had to be put down,) it seems fairly obvious after a few days. The bird will have less energy, may make a popping/crackling noise when breathing, and may have other symptoms (diarrhea, discharge from eyes/nose, etc.)
I can be a little bit of a hypochondriac too when it comes to thinking my pets are ill....but I try not to jump to immediate conclusions now. Harm can be done if you give an animal medication when not necessary.
How are your birds doing now?