A couple suggestions:
(1) ask them to supply their best program/tool that displays their
knowledge
(2) have some predefined task that the employer creates to test their
ability, then give them x amount of time to create it.

Not sure how else you would go about it.

mctwist wrote:
Hi, I have a client which is currently hiring, and asked me how he
could test a potential employee on his/her Excel skills. I've never
been asked for such a thing, nor I know where to look. Anyone has any
idea?

Thanks