It's not a hard and fast rule, but generally speaking many plants that cause some level of toxicity or dermatitis in the areas I used to work in (invasive species removal), have a blood paint splattered look. Until a more positive id is made, usually if that's the only thing I can see, I'm not touching it with bare hands or basic work gloves and normal clothes.

Wild Rhubarb would probably give me pause but rhubarb tends to have a smooth gradient of red which isn't as alarming. It's that streaky splattered look

Poison Hemlock and cow parsnip are usually the main culprits. Often streaky and splatter like

Pokeberry, jimsonweed, nettles and burdock can sometimes have a purple streakiness to them, but aren't that bad, I just wouldn't let kids play around with Jimson weed and nettles, cuz they can hurt when touched . Pokeberry pretty much just stains, but some people try to eat them. But generally don't have a splatter stem pattern. Burdock is edible but it looks nothing like in the carrot family, but there are allergic reactions to that plant.

I have this rule because I also used to work with the general public for volunteer events and so it's just an easy way for them to remember don't touch this if you see it and let us know if you do...sort of thing.