In this photo you can clearly see the differences between the Western Grey Kangaroo and the Eastern Grey Kangaroo.

The Western is on the left - pay close attention to the ears, they are densily furred at the base and nearly furless (or fine fur) at the top two thirds. This is the easiest way to distinguish between the two species

The Eastern is on the right. Note that the ears are completely furred.

You can also see the colour differences, the Western is commonly browner than the Eastern.