How does one determine the luster on a coin that has orginal, dirty surfaces?

Longacre

Suppose you have a gold coin that has never been dipped and the sufaces are original (a lot of people use the term, "dirty" surfaces). How do you check the luster on the coin? Will the luster shine through those original surfaces, or will the coin appear to have uniform dark surfaces? Supposedly you are able to see luster in the "protected areas", such as around the stars, but how should you hold the coin to see this, and what lighting should you use? Should you tip the coin back and forth under a stationary light?

dragon

Good luster will generally be evident through all but the heaviest toning and/or very dirty surfaces. It may sometimes be necessary to tilt the coin under various lighting on heavily covered surfaces.