Outright forgeries are not a concern with these issues. However, forged perforations or reperfing is a problem, particularly with the Standing Helvetia issue. Second are fake cancels applied to stamps that are more valuable used, though likely not a problem with any of the above.
It is most important to identify these stamps properly by watermark and perforation.
hy-brasil, I understand your point ! Actually the sitting helvetia issue has differences on the paper (white paper till 1881 and the latest issue 1882 granite paper), what I concern is the 40 rappen "strubel" how you can tell if it is an issue of 1854 or 1858?
For the 40 rappen, the 1854 stamp is on thin paper, the 1858 stamp is on thick paper; that is a main test. The difference is pretty clear, with the thin paper quite flexible. The Sitting Helvetia issue with FRANCO comes on thin and thick papers and I think provide a good paper thickness comparison if needed.
I also believe that the 1854 and 1855 Strubels are a much brighter and paler color than the 1858 ones. Yours looks like the 1858 issue.