I go to a commercial lab that is based on Texas. They sell you a pack of swabs for about $20 and then when you need to test a chicken for something (Marek’s, MG etc.) you swab the chicken’s throat or vent (or both) and cap it off and send it to them.
Their website lists all the tests they do and you can call them about others. I think Coryza wasn’t on the list but they can do it.
This is the lab I have used. I have had a good experience with them several times but I am sure there are other labs.
Some State labs in the US will do testing as well.
https://www.vetdna.com/