no wrinkles around the ends and the fresh ones have very thin skins.

Most of what you find in the regular grocery stores, e.g., Giant or Safeway are actually sweet potatoes. They should be firm, dry, the skin may have some blemishes but avoid buying ones that are soft or obviously cracked:

http://www.huffingtonpost.com/2011/11/16/difference-between-sweet-potatoes-and-yams_n_1097840.html

I love them roasted as fries or mashed.

I'm no authority on the difference between this orange one and that one, or between sweet potatoes and yams.

But I do know one different one--the ones that are kind of purple on the outside are yellow, not orange, on the inside. And they're sweeter.

DS loves them. I think they're called Japanese sweet potatoes.

How should they look?