Apparently undermethylation can be diagnosed using blood work. I googled it a bit, and what I saw seemed to suggest that it isn't widely accepted in the medical community.
I'm wondering how legitimate this diagnosis is. That's hard to define, but I suppose that would mean "has been discussed in peer-reviewed articles in legitimate journals," among other things.
Also, to clarify: I don't doubt that it's a thing. I guess to put my question even more precisely, it would be "is it considered legitimate to care about it and attach weight to it."