In the earliest stages the signs can be subtle - a slight wheezing sound when breathing, resting in a head-up position against one wall, and yawning more than normal. If she'll relax when you pick her up, you can also try holding her so that your hand is under her lungs, sometimes you can feel a "gurgling" when they breathe. Mainly it's you recognizing that there's something not quite right based on how well you know her. For instance, one of my adult BRB's constantly wheezes, hisses, clicks, puffs, etc. when he breaths, especially if he's exerting himself, and he's done so from the first day I had him. No RI there, but the same symptoms in most of my other snakes would mean a vet trip.

I agree that if you even suspect a problem she should be isolated and monitored.