I've benn corresponding with Odile Madden, the primary scientist on the NASM study. She says,
"Raman and XRF are unlikely to give you any useful information about the date of the PMMA. Raman could tell you it’s PMMA, and XRF only identifies elements in the periodic table that are heavier than, say, silicon. PMMA is just carbon, hydrogen, and oxygen. If you had a sample of window from a contemporaneous 1936/7 Electra to compare with your sample, it might be possible to do some trace element analysis."
So we're back to destructive testing, provided we could get our hands on an original sample to test. But there's another question to answer. Were the cabin windows on NR16020 really Plexiglas? The engineering order calls for "shatterproof" which we always took to mean Plexiglas, but it might also mean laminated safety glass. How could we know? If we had a surviving Lockheed 10 built after January 1937 that still had the original windows, Raman could tell us if they are PMMA, but I'm not aware of such an aircraft. The Electra at the New England Air Museum is only three serial numbers from Earhart's, but its windows were all replaced during restoration. The same is probably true of all restored Electras. The old windows tend to yellow and craze so they get replaced.