How Can I Spot Signs Of Cleaning?

sometimes, cleaning (scrubbing) is obvious. a lot of hairline scratches running parallel to each other, or polish marks on very shiny flat surfaces, with grime in the little crevices.

I have a half-dime that is in beautiful condition, has two tiny hairlines and one bump, an nothing that looks like evidence of cleaning to me. but dealer says the original luster of the coin is gone, which indicates cleaning. I see a beautiful (if tiny) silver coin that has touched very few hands, and should command a high grade.