Signs of an RI are excess mucus, gaping mouth, bubbling at the mouth, sounds coming from the throat (clicking, popping, whistling, etc), wheezing, "stargazing", and puffy throat.
Sometimes you also have preshed or stress whistles as well. They sound similar. Sometimes if the snake is stressed when being handled it will make a whistle, and also when they are going in to a shed, the excess skin in the nostrils creates a whistle.
Its not uncommon for snakes to hiss a lil and puff some air when handled, they are just letting you know they don't like being handled.