What helps me to recognize the purls from the knits is if I'm working in stockinette stitch, and I can see the right side of the work while it's hanging to the right on my needle, it needs to be knit. If I can see the right side of the work hanging to the left side of my needle, it needs to be purled.
<<<<<<<<< This needs to be knit, because the right side of the work is facing up while sitting to the right of the needle.
<<<<<<<<< This needs to be purled, because the right side of the work is facing up while sitting to the left of the needle.