It depends on what it is. If it's something like math, I'll look it up online and then ask the professor to take a look at it before it's due to tell me if I'm on the right track. If it's something like a type of writing assignment that I've never done before, I'll send the professor a sample of what I have so far and ask is this what you're looking for, or should it be more like A, or B? They can answer you pretty easily if you phrase it like that, and then you shouldn't be so stressed out.