Are you doing a repair?

Varnish is noticeably softer than epoxy and as Richard wrote just wet sand. You will cut through varnish easily and a whiteish water will be going on. Once you hit the epoxy, you will think you need to replace the sand paper because nothing will be happening. Its like sanding stone. If you're worried if epoxy will stick varnish residue, it does. I've done plenty of repairs on my boats and others and never had an epoxy glassing job fail and I never really know if I got through all the varnish.
Besides, there always the ends of the repair with you are feathering into varnish.

If you have some thin epoxy areas, just hand sand them with fine wet paper.
Believe me, once you hit hardened epoxy you will know it.