Hi,
The best giveaway for detecting VoPo's is the occasional mismatch of small parts, usually with the numbers crossed out. The newly manufactured replacement barrels, found on many of them have a lone crown/N proof on the bottom and no serial number or other markings, except for the random '9mm luger' or '9mm para' marking for some export markets. The crown of the barrel is always a bit in the rough, not smoothed.
Other giveaways are the wellknown VoPo grips. Many of them have police proofs (or have them peened out), many of them lack any form of DDR proofing.
Magazines: Haenel/Schmeisser type extruded with a /1001 marking on the tube. Base is usually made of pot metal, not aluminium.
Loading tool: Unmarked, thumb piece has horizonal anti-slip pattern.
When you're in Germany, try to locate a book with a blue plastic cover called 'Kleine Waffenkunde', published by the East German ministry of interior around 1966. It has some excellent VoPo illustrations in it.
The russian capture lugers are identified by the large X mark stamped somewhere on the pistol. They usually lack their original grips and mostly have cheap plastic replacement grips.