Check the app's description. Otherwise, if the applications graphics are fuzzy looking/ blurry, then the app probably doesn't support the Retina Display.
As per my point of view we already know that Apple iPod Touch 4G may feature dual camera with FaceTime support. Now there is news that iPod Touch 4G Retina Display might be reality.
If the rumor is true (and it probably is), that means developers will need to start getting their apps updated, since they'd have more hardware to support.