This is very much not the case with mushrooms, most people who've accidentally eaten a deathcap (Amanita phalloides) have reported that they're delicious. Flu agaric (Amanita muscaria) which can be detoxified by boiling it and changing the water multiple times, is pretty darn good. I think it's better than the average grisette (the non-toxic Amanita sect. vaginatae spp.).

Ok the other other hand, the destroying angel (Amanita ocreata) is said to taste pretty bad.

For a non-amanita example, I've spit-tested the toxic Agaricus deardorffensis and I thought it tasted pretty good. That one is an odd case though since some people are unaffected by its poison and it's possible that's correlated with not being able to detect the unpleasant sharpie-like odor it's said to have, but I wasn't willing to give myself the shits for science so it remains a mystery.

It's how we know! Just put it in your mouth and let us know.