Forgeries are noted in the catalog.
It would be difficult to tell accurately without seeing them and comparing them to a known source.
My rule for stamps that might have been forged is to assume they are unless there is compelling evidence they weren't. If you didn't pay too much for them they were probably forged. If you bought them from a reputable source who can attest to their status as legitimate - then maybe they are not forgeries. So hopefully you got them in a mix and didn't pay too much for them.
re: British North Borneo #36, #41, and #43
Forgeries are noted in the catalog.
It would be difficult to tell accurately without seeing them and comparing them to a known source.
My rule for stamps that might have been forged is to assume they are unless there is compelling evidence they weren't. If you didn't pay too much for them they were probably forged. If you bought them from a reputable source who can attest to their status as legitimate - then maybe they are not forgeries. So hopefully you got them in a mix and didn't pay too much for them.