A boyscout and frequent camper in my youth, I always had trouble recognizing poison ivy. I thought I was dense, but over time have learned that it’s because the appearance is just too damn variable. To be safe, of course, avoid the 3-leaved plants. Problem is, there are a lot of damned 3-leaved plants.
This BW shot shows the shape I learned as a boy, but the edges can be smoother than this, too.
The toothed form is merely a variety of the ivy, and is often mis-ID’d as poison oak.
I read on a website that the oil in the poison trio, ivy oak and sumac, is easily oxidized to a black substance, so if you damage the leaves they will turn black at the damage point (where smashed or torn). I’ve never tested this.