The first 2 airmail stamps of Switzerland routinely come up in the common auctions. The are for the most part fakes unless accompanied with a recent certificate by a Swiss expert.
Genuine Issues
1. The curvature of the wings
2. The separation width between the feathers.
3. The exact shape of the openings in the central disk
4. In particular the angle of the last bottom feather
5. The overall angle of the overprint
I have not seen any full forgeries as the base stamps are relatively easy to find.
Kull made large quatities of these fakes.
Fakes
This is a very crude fakeThe wings have little curvature
The openings between the feather groups is too large
The feathers are too close together
The openings in the center are too small
The overprint angle is wrong
The gaps between feather groups are too large
The central openings are different
The last bottom feather is too inclined
In particular the feathers are too wide particularly in the 50 value
30c Fakes
The used 30c is rare and an excellent copy will fetch over $1000Here the forgery on the left is compared to a certified genuine right.
The feathers are not curved enough
The feathers are too close together
The central openings are wrong
The last right feather is little more than a blob.