I've done plenty of repairs and to be honest, you never really know. Varnish is soft compared to epoxy and you will sand through the varnish easily. I always notice that the varnish sands white and comes right off then you get this stubborn layer thats like sanding stone. I suppose you could dissolve the varnish with acetone, let it dry and sand but unofficially, I would just use common sense and if there's a few traces of varnish, the epoxy will stick fine. If in doubt, just sand a little more.

The bottom line here is, you're never really going to know.

I will caution you that if you sand into the wood, there could be a slight color difference when you wet it out. You can test this by just wetting it with some water. On repairs I have done, I will sometimes add a tiny bit of artist oil paint into the varnish to stain it a bit if needed. There's no one answer to this other than some experimenting.