The damage will tell you. Voles move above ground, and cause more visible damage than moles. If you see obvious "trails" above ground from hole to hole, that's a vole. If you're just feeling the ground get squishy, and seeing the holes in the area, that's a mole. Voles are harder to get rid of, in my experience, and do more damage.
Whatever you do, please don't poison them. When they're dying, they sometimes flush out of their holes and a hawk or owl will eat them, which will poison them as well. The mole traps haven't been super effective for me because they don't do anything to deter future moles, they'll just kill one at a time.
These in conjunction with the sound deterrent spikes work well for me. Disclaimer: I do not work for any kind of pest control or product company!