Difference Between Respiratory Infection Symptoms & Reaction to Dusty Environment

Recently, I had to move my flock of 5 (4 hens, 1 rooster) into our barn area because it finally decided to be winter here. With this move, my rooster has started to develop a runny nose and sneezes sometimes. These are the only two symptoms he has (no rattling in his breathing, no lethargy, no lack of appetite for food or water or the hens). This has only been going on since Christmas and their move into our incredibly dusty barn - in fact, the area we live is ALWAYS incredibly dusty, unless it's currently raining/snowing.

My question is, how can you tell if sneezing/snotty nose is just a reaction to the increase in dust/dander from horses/alfalfa or if it's the beginning of an infection of some kind? I have ordered some antibiotics for respiratory problems just in case, but I obviously don't want to use them if I don't need to. I'm also currently searching for a vet in my area that takes care of poultry, but it's been a fruitless search thus far.

Any suggestions/advice would be welcome (or if this is covered somewhere else on the site, a link would be great - my search fu is lacking lately).