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If you were in the field, the most useful thing to differentiate the two would be knowing where you were (the ranges don't overlap). From a strictly visual perspective, the base colors are USUALLY different - with the Eastern tending toward yellow or olive, vs the Western's grey. Both species can be brown,though, and, at a quick look, those specimens are a bit more difficult to differentiate.
Both species have a post-ocular stripe (diagonal)...the Eastern's tends to stand out more due to the stark contrast with their lighter colored face, while the stripe on the Western usually blends in better with their coloring. Once again, though, on darker colored Easterns this becomes less reliable.
Both species also have tail bands; however the Western's black and (off)white bands are very distinct, whereas the Easterns black bands are on a darker ground color that often disguises them...especially at a glance.
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