I'm a relatively new NP, but usually if I'm stumped, so is my collaborating doc. I mean I do ask their opinion on occasion, sort of run my thinking past them, for maybe stubborn blood sugar or BP, or maybe if some infection didn't clear up with first round of ABX, but that's probably because I'm still new.
Most of what I see are the same things: high cholesterol, high BP, diabetes, bronchitis, colds, coughs, strep throat, chicken pox...things that the doc sees too. Sure, once in a while a rash looks odd or the symptoms aren't as cut and dry as the text book, but I usually have a good idea what it is and at least have it narrowed down to 2, maybe 3 differentials. This is the same thing my collaborating docs do. If I'm stumped, they usually are too and then after discussing I refer to a specialist just like they do.
Keep in mind, NP work, or MD work for that matter, is not like an episode of House. Most people have "normal" things wrong with them. The saying is: If you hear hoofs; think horses, but keep in mind zebras exist. So, rule out the life threatening and work them up for the usual things, if they need a specialist refer to specialist.