I cannot see it.
You can feel it as a sticky spot with your hand.
If you try to sand it will show up as a quickly clogged piece of sandpaper with what looks like something waxy (with embedded sanding dust).

I sometimes get it off with a knife blade by scraping, but its incredibly tedious, and a waste of time.

Just wash every surface that is completely cured with water and a scrubbie. Not hard pressure, just get every square inch scrubbed and well rinsed. No detergent or solvent required - although I most often use a mild ammonia/ soap commercial cleaner, just because I have it.

One of the worst situations to me is when I don't get the weave of the glass filled before I let it fully cure. If there is blush down in the weave pockets it can be difficult to get out and impossible to see. You will only see it when you put a layer of epoxy or varnish over the blush and it shows up as little yellowish dots regularly spaced.

Just wash everything and don't go looking for problems. :)

IMHO