Toxic people try to control you. Strange as it might sound, people who aren't in control of their own lives tend to want to control yours. The toxic look for ways to control others, either through overt methods or subtle manipulation.
Toxic people disregard your boundaries. If you're always telling someone to stop behaving a certain way and they only continue, that person is probably toxic. Respecting the boundaries of others comes naturally to well-adjusted adults. The toxic person thrives on violating them.
Toxic people take without giving. Give and take is the lifeblood of true friendship. Sometimes you need a hand, and sometimes your friend does, but in the end, it more or less evens out. Not with the toxic person — they're often there to take what they can get from you, as long as you're willing to give it.
Toxic people are always "right." They're going to find ways to be right even when they're not. They rarely (if ever) admit when they've messed up, miscalculated, or misspoken.
Toxic people aren't honest. I'm not talking about natural exaggerations, face-saving, or white lies here. I'm talking about blatant and repeated patterns of dishonesty.
Toxic people love to be victims. The toxic revel in being a victim of the world. They seek to find ways to feel oppressed, put down, and marginalized in ways they clearly are not. This might take the form of excuses, rationalizations, or out-and-out blaming.
Toxic people don't take responsibility. Part of the victim mentality comes from. When the world is perpetually against them, their choices and actions can't possibly be responsible for the quality of their life — it's "just the way things are."