From:  http://www.wikihow.com/Identify-Poison-Ivy

The only poisonous plant in our area is Poison Ivy, which identified by:

• Green leaves of three
• Vines often grow up on sides of trees and  appear "hairy".
• Thumb shaped wedge on leaf (but not always)
• The leaflets are broad, and the two lateral (side) leaflets are smaller than the terminal (end or middle) leaf.
• The center leaf usually (almost always) has a small stem, wheres the two side leaves grow directly from the vine and do not have small stems.
• The leaves tend to be a bright to dark, waxy green when viewed from above. When viewed from underneath, they appear lighter and fuzzier.  In spring, the leaves are usually a bright green, while in fall, they turn red (poison ivy) or bright red or orange (poison oak).