Check for drooping leaves low on the plant. Spider mites feed by piercing the bottom of the leaf and sucking out the contents of the plant's cells. This causes the leaf to lose moisture and makes it droop.
Turn the leaves over and inspect the bottom for patches of white or yellow spots. These are areas where the chlorophyll has been removed from the leaf.
Look for webbing on the underside of the leaf. Spider mites spin webs to protect themselves from predation from insects.
Tap a suspect leaf over a white piece of paper and look for small moving dark spots. These are mites that have fallen off the leaf.