I have a worker who is lying about changing diapers. Any advice?

Have you told the worker that you're marking this persons diapers, so you know in fact that they're lying? If not, just come clean on that, and see what happens. You can also set up some inexpensive security cameras that will be your witness to the non-changing of the diapers as well. As the saying goes, a picture says a thousand words.

Assume you mean adult briefs when you mention "dimpers" 3x. Also assume these briefs are not wet or soiled and in need of being changed when you get home and find the same brief on your loved one? Or is it wet and soiled, because that makes a difference?

If you are distrustful to the point you're marking briefs to catch this caregiver in the act, then I'd say fire them, unless said brief is dry/unsoiled and then you're just looking to nitpick.

What do you want to do? Fire them? Give them a hard time about the lie? Are they private or from an agency? Get rid of them ASAP.

Fire the CG and hire another one.