How do you identify poison sumac out in the wild?

Not easily! Poison sumac is not very common. It grows only in very wet areas. Identification is not easy. The leaves are smooth not jagged or hairy and there are 7-9 leaves per stem.
Leaves of three... Unfortunately poison sumac is not as easy to recognize as poison ivy and oak. Luckily it is rare in the north and completely absent in many states where it is often confused with other non-toxic sumacs. Here is a link i found that might help. Though i don't recommend it, one way is to expose a small area of skin and observe for reactions over 3-7 days.