I think pastel lessers are easy to differentiate when looking at the head. Although to be honest the pastel lesser pictured above just looks like a lesser to me. I guess some are harder than I thought.
It also depends on the quality of the pastel lesser. Some are very easy to tell and others you just have to believe the breeder. I have seen some posted online that I do not believe are pastel lessers. Usually the head is the best indicator.