Some of the most common overprint forgeries on the First Essayan Pictorial Issue can (again) be bought at Cherrystone.
This forgery of the 35 Gold-kopeck overprint is quite crude and easily spotted. You can check here for my article with in-depth information.
Here the overprint in detail.
The second forgery is on the 25 000 ruble stamps. The forged overprint can easily be spotted by the rounded inner shape of the “0”. If you check the stamps in your collection this is not hard to see.