It's time again to refresh our memories about lizards! The Greater Earless and Zebra-tailed lizards look similar and both wag their zebra-striped tails. Look at the two bands on their sides. If the bands are near the groin (see above) remember: G for groin, G for Greater Earless. If the bands are near the armpit, remember: A for armpit means A Zebra-tailed lizard. In general, you'll see Zebra-tailed in the lower canyon (visitor center to dam, roughly) and the Greater Earless in the upper canyon; but they don't always stick to their areas.
I recommend this site: Reptiles and Amphibians of Arizona, for those interested in more info about these amazing animals.