Depending on the age of the pups, it can be tricky. When I was breeding strictly for pet/show, I found the best age to judge pups was at 3 weeks of age, then again at 12 weeks of age. In between those times they went through awkward stages where nothing was proportional or fit right.

Dumbos are tricky because the shape of their ears gives their head a different appearance. Are you looking to actually breed to show standard? If so, you want to be very careful to avoid "banana nose", also known as a Roman or convex nose. Look instead for a wedge shaped head that forms an equilateral triangle when viewed from above. Since pups go through an awkward stage, look for the shortest nose you can get, that will even out as they grow, whereas a long nose will probably just stay long. Show quality ears are open, flat and round. The tulip ears are actually discouraged in show standard, as are pointed or "elf" ears and of course small "teddy bear" ears. Watch the eyes too. There's been a tendency for dumbo to end up with small eyes. Look for large, bright, round eyes, the largest and roundest you can select.

If you aren't actually breeding to show standard, then none of this really matters too much, just select what you like best over all.