I think toxic is relative; what is toxic to me might not be toxic to you, and vice versa.

Maybe among the signs of a person who is especially toxic to us individually though, would be clear indications that they are aware of our buttons and push them anyway, in order to cause actions on our part with negative consequences, such as becoming angry, or feeling especially intimidated or embarrassed, or generally feeling lessened in their presence instead of enriched. Or even invoking our desire to be helpful and giving but to our eventual detriment.

Such as, there's this whole culture of "negging", which is when guys who want to prey on girls with low self-esteem specifically say things to invoke it, to imply that said girls are not good enough for them, and cause them to aspire as a result. Which I think is pretty horrifically toxic behavior. Luckily, I'm fairly impervious to that particular toxic behavior, which only invokes in me a mild rage; not the intended result. I pity the po' fool who tries to neg me, for I do not find it to be the least bit cute.

But I have a couple of other weak points; less than I once had. Fewer and fewer as I get older.