Elm has distinctive wavy bands of small latewood pores in cross section. Slice a small section with a razor blade and look at with a low power lens (10x is perfect)

ELM:
wood-elm.jpg

The rays on oak are wide and distinctive on cross section, very long and distinctive on longitudinal radial and tangential sections. The large pores in white oak are filled with easily seen tyloses, making them look kind of sparkly. You can find end grain photos on the Wood Database.

JKJ