Since I can't be there to see/hear what is happening, my rule of thumb is to go with the parent's gut instinct. If it is something that seems outside of the normal range to you, then it is a problem.
I would check for things like excess mucus production. She is too young to tell you that she is congested. And if that is her constant state, it may feel 'normal' to her. But clues to it are lots of saliva production and loud breathing as well as the noises she is making. Also making noise during eating would be a clue. My little guy had lots of fluid behind his ears that made swallowing a painful experience. If you aren't seeing a runny nose, that doesn't mean there isn't any congestion. If the mucus is very thick, it can be stuck deep in the sinus cavity and leave just a small passageway for breathing.
I would recommend lots of fluids -- maybe some marshmallow tea -- and eliminating mucus producing foods from her diet. Monitor her response, and you'll know if you are on the right track. The dietary changes should reduce the amount of mucus. The marshmallow (the herb, not the candy) tea will help her get rid of the mucus. Lots of fluids will help thin the mucus. HTH