You can definately tell a hiss, it's a loud sucking of air and then releasing it. It's a lot more sharp than just normal wheezing. Best thing I can relate it to is a really upset cat, that loud hissing noise they make as a last resort right before they swat you. But with a snake they tend to get louder and prolonged the more they are bothered, even opening their mouths. Some wheezing is normal, a lot of snakes make a very slight whistling noise if they are tired of being held, struggling to climb or lever themselves, etc. I wouldn't be too concerned unless you see actual bubbling or drooling from the mouth or nose. And even then sometimes you can distinguish between mucus (it's thicker and more stringy) and leftover drinking water. And if you ever do see beginnings of an upper resp. inf. you can sometimes actually relieve it with just raising the temps. But I'm not saying by any means to avoid a vet trip if you think it's serious enough