From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156717-934633
Question: How do we tell whether we have moles, voles, or gophers
digging in our yard?
Answer: Moles create characteristic mounds and tunnels that rise above
the soil level. Although the tunnels aren't always visible, you can
usually spot their mounds of soil with a hole in the top. Voles are
tiny creatures, and are sometimes referred to as field mice. They
don't dig tunnels or make large mounds, but rather have trails through
tall grass. Sometimes voles use mole tunnels to feed on roots and
bulbs. If you see fan-shaped mounds with a hole near the edge, you
probably have an infestation of gophers.