Yep, file fish. Just browsing the categories at live aquaria will be helpful in recognizing at least what type of fish you're seeing. Damsel, wrasse, filefish, angel, etc all look a certain way that you'll learn to recognize. Then you're at least in the general neighborhood to narrow down what you've got. I have one of those pocket guide to reef fishes and it's handy to have.
But overall, I agree, ask the LFS what it is and do your research so it's a good fit for your tank.