I order 99% of my flavorings, most all TFA, from Wizard Labs, and they state that none of their flavorings contain diacetyl, which is a main diketone many used to be concerned with. As far as the other diketones...on each page for their individual flavorings, it shows the Ingredients List which contains most contents like PG, Art. & Natural flavorings, etc.
Then under that...is information from the flavoring manufacturer (ie. TFA). If either or both of the other 2 main diketones (Acetoin & Acetylpropionyl) are in that flavoring...it will tell you there. Usually, they'll be found in some of the custardy note flavorings, the creamy type ones.
They DO also carry what's called the "DX versions" of many of the regular version s that do have either one or both of those. For example, they carry a regular Banana Cream (with 1 or both of those in it) AND they carry the DX Banana Cream with neither of those in it. I just usually always order the DX versions myself.
The TFA website itself will also tell you what each flavoring contains. So that's another source.
By the way, Wizard Labs also carries Nic products, as well as VG, and PG in quantity. They've all been good from them, IMO. You could save yourself some S/H by ordering more things from 1-2 places instead of all 3.
I hope that helps!