Here's a run-down of the tell-tale differences between real MAC eyeshadows and the kind of fakes often sold on Ebay. The fakes were bought, by me, in good faith, so yeah - I'm a mug. But you get to benefit from it!
The fill weight of 1.3g only ever seems to occur on a genuine shadow. 1.5g and 1.3g fill weights are found in genuine products, however the fakes tend to be marked as 1.5g. So while a 1.5g weight alone does not indicate a fake, if your shadow looks dodgy anyway, a 1.5g might be a further sign of counterfeiting.