Depends on when the guns in question were made. Simson was an independent Jewish-owned operation before WWII. Taken over by the Nazis. After WWII, when Germany was divided East and West, Merkels and Simsons were indeed made in the same factory in Suhl. That being said, German guns in general--both OU and sxs--show many common features no matter who made them. There are certainly exceptions, but it can often be hard to tell a Merkel from a Simson from a Sauer just at a glance from a distance.
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This statement only applies to the post war guns. Pre WWII, they are relatively easily distinguished from each other, especially Merkel from the other two.
