I took a close look at several lights with the XM-L led and the new XM-L2 to see if there was a way to tell them apart. I found there is an easy to spot difference. There is a 5mm square board the the emitter and dome are mounted on. For the XM-L that square is green and on the XM-L2 it's silver.
Mtbr's 2016 Winter Biking GearReviews and Roundups
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Thread: How to tell a XM-L from a XM-L2