Giant ragweed bears a "palmate" leaf. That means its shape resembles that of the palm of a hand. I suppose that description is somewhat more apt for one of the two leaves shown in this image than for the other.
For, as you can see from the image, there are two leaf types for giant ragweed: one type has five segments (and therefore truly reminds one of a hand), the other three. Don't let this discrepancy bother you excessively in your identification efforts: the fact is, if you find a very tall weed with leaves like either of the ones pictured above, there's a good chance you've encountered giant ragweed.
There are other types of Ambrosia besides common ragweed and giant ragweed. According to the University of Tulsa, there are 21 species in North America. Ragweeds, generally speaking, thrive in dry areas, so it is not surprising that there's even a type found widely in the deserts of the American Southwest. That would be burroweed or "bur sage" (Ambrosia dumosa). Desert plant or not, this type of ragweed is just as problematic for allergy-sufferers as are other kinds.
Finally, on Page 7, I'll show you what the flowers of giant ragweed look like....