Here we have a cake of menghai 8582 on the back side there is a seal which under normal light looks like it uses techniques used on currency to prevent counter-fitting but they go a extra step further. Run that seal under a UV light, and watch. If genuine your seal's "Dayi" emblem should light up yellow some other versions actually have a square that fluoresces with the word "DAYI" in it. This is also the case for the seals actually on the tea cake. Lets take a look.
Seal under black light:
Seal on cake under normal light:
And that folks is one of the ways to tell if your Menghai is authentic.