Well, helmets for example. From wat I've seen, the M-15 Adrienne and the 1926 models don't look all that different with the exception of the Colour and the badge.
The model 1915 and 1926 Adrian helmets look quite different. The only part that is really similar is the crest on top. The 1915 type has a fat line running between the visor and the shell where it was welded together, and lacks visible rivets on the exterior due to the different internal lining system. In addition, the 1926 model is more elongated, while the 1915 was more circular.
That being said, I imagine that other than the circular shape all the details could be differentiated via the textures.
About a quarter of the troops in 1940 still wore 1915 helmets, but repainted in the olive drab color. Paradoxically, about three quarters of the Adrian 1926 helmets still used the 1915 badges painted in olive drab; the round type introduced in 1937 that you see everywhere now only became very common during the Vichy regime, which is even more ironic when one realizes the symbol "RF" no longer even made sense.
Other things to beware of:
-Proper Adrian 1926 chinstraps in 1940 used brass rivets. The aluminum riveted chinstraps you see on sale everywhere weren't introduced until Vichy.
-Until 1940, the Adrian 1926 liners were dyed black. Probably beginning in 1940, they were left in natural brown (some collectors would argue that only black liners are proper, but I have color photos of captured troops in 1940 with brown liners). Most Adrian 1926 liners on sale today are brown (not even I own a black one; one of mine is postwar, the other is undated), and most are postwar, but in 1940 almost all should be black.
