The only way to know for sure is to ask. Have them list four or five things that really motivate them. This can be done in a staff meeting, by e-mail, or one on one.
What if you don't have employees? Decide on the four or five things that motivate you. Take them to your manager--he or she may be in the dark about what fires you up, just like you may be in the dark about your own employees.
Source: "101 Ways to Have a Great Day @ Work," Stephanie Goddard Davidson, 1998.