What is the best way to check for a respiratory infection?

I don't think any of my snakes are ill at all, and none have ever even had a health issue, but I'd like to know for future reference what, besides watching for mucus or discharge around the nostrils or mouth, is the best way to check for a respiratory infection? I figured I would just ask since you guys seem to have seen it all by now, haha.
Winter is coming and I'd like to know all I can possibly know, just in case. We have really freezing winters here, and if the snakes lungs are as fragile as mine, then this is information I should probably write on down in the snake journal

Any advice?

You can sometimes hear a clicking or wheezing noise as they breathe. However, don't diagnose RI based solely on this one symptom, as it can also happen just before a shed. The skin loosens around the nostrils first, causing irritation. The issue clears up as soon as the shed is finished.

Right, excellent. So I'll just write down that I should be listening for wheezing ALONG WITH watching for any mucus discharge around the mouth/nostrils.
I have a snake vet, of course, but I like to know all I can possibly know, especially since right now, it's looking like it's about to be an extremely cold winter. Even though my snakes are in great health right now, I want to be prepared for anything.
Thank you!

I wouldn't sweat it. As long as you have the proper heating setup in your Corn's tank, the ambient air temp (either inside or outside your home) will never be a problem. Your Corn will always have the correct temps.

I had a big, ugly wild caught black/yellow hybrid with pneumonia once and he was actually breathing through his mouth. So wheezing, stuffy nose and mouth breathing would be pretty good clues.

Thank you. I'm not really all that worried at all, I just figured this is something I should know, especially this time of year. And trust me, the heating setup is great and like I said, they've never had any health issues. It better be great with all the trouble they gave me, haha. I had to go buy a second freaking thermostat because my hatchling and my yearling each prefer drastically different "hot spot" temps!
They're brats, but I love them.