Moved from Unidentified Leaf Mines.

Distinguishing window feeding (1 epidermis intact) from leafmining (both epidermises intact) is usually as simple as looking at both sides of the leaf and seeing whether tissue has been removed from either surface. If you're not sure after that, hold the leaf up to the light--if there is anything inside (frass, and/or a larva) that you can't see from either surface, it is a leaf mine. If you're still not sure, you can try peeling it open: if it's possible to peel it open (vs. just ripping a hole through the leaf) then you've got a leaf mine. Window feeding also tends to be broken up into separate areas, where as a leaf mine will be one continuous trail or blotch. It's very rare for window feeding to produce a linear trail like the mine in this photo.