Answer: This is hard to explain but, the poison ivy leaves in a set of three have the center leaf on its own separate little length of stem. The Parthenocissus leaves (Virginia creeper and the related Boston ivy) all start from the same central point. Here is a diagram you can look at -- admittedly it is hard to see the difference in a drawing. Also, the poison ivy has a hairy-looking stem. You may need to cut and paste the complete url to make it work correctly.
Here is a color photo comparison of the two (scroll down the page a little)
Probably the best thing to do is study photos and actual plants that you have already specifically identified, soon you will just recognize them -- the coloring is a little different, the placement of the leaflets is a little different, the vine and growth habit are a little different. I hope this helps!
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