had a pair of Uggs

The imprint on the bottom of the boots should be identical on both boots - There is usually only one curved line on the heel of the sole with one curved and spiked line above it;UGG Bailey Button Triplet 1873 Boots Grey will have 2 or more of these curved lines (see image).
Both imprints on the soles of the boots should be symmetrical. Fake boots will more than likely be off centre and alters slightly from the opposite boot.
The heels of genuine Ugg boots should not wear down easily. I have had a pair of Uggs for over 2 years (very regularly worn) and the soles are only very slightly worn down at the edges.