If QCD is to undergo a second order phase transition, the light quark retur
n probability is universal for large times at the critical point. We show t
hat this behavior is distinct from the one expected at the mobility edge of
a metal-insulator transition or a percolation transition in d less than or
equal to 4. Our results are accessible to current lattice QCD simulations.
(C) 1998 Published by Elsevier Science B.V. All rights reserved.