Unless you are very unusual and are completely off the grid, you've probably solved the question of how you personally decide how to trust a wide variety of commercial agencies of highly varying degrees of regulation and highly varying degrees of life-threateningness. Possibly without realizing it. In theory it may be unsolvable, in practice it doesn't seem to be.
It should be pointed out that this isn't theoretical, either. People die because the FDA forces them to not take drugs, even when death is essentially assured in the short-term anyhow. It's a well-known problem. If the FDA was more advisory, I suspect it would be a net good. I really can't imagine people en masse running down to WalMart and buying Dangerousol (now with twice the danger!) any more than they manage to now. (Which you can't ignore either, the FDA is not perfect.)