Hey Pop,
What model of depthfinder are you running? Is it color or grayscale? On my color units, i could tell if it was crappie or leaves by the arch color. My arch would be red with just a smidge of yellow in the middle. On grayscale, its sometimes harder to tell but ocasionally you will see a lighter shade of gray inside the arch. Remeber, crappie are more solid and present a larger target than leaves. The best way to learn this is when you catch a fish, make sure he's hooked good and let him back in the water while he's still on the jig and lead him around underneath your transducer. Then you can sort of see what kind of return you get. The exception to this is small baitfish. If they're not grouped in a tight ball, they will give the same kind of return a leaf will give. Hope this helps!