Another pointer, according to Chervenka, to copies is that the glass behind the cameo was ground out from inside the vase behind raised parts, whereas the original will be smooth.
I must admit to being puzzled by this but it presumably improves the light transmission.
Other warning pointers of Chervenka are
Clear glass body with paper thin flashing.Spattered or mottled body (needs a bright light through the glass to detect)No ground pontil, sheared rimMark prominent in top half of piece.Concentric rings in the ground pontil of originals. Due to the thickness of outer casings.Chervenka makes no mention of the high quality Japanese examples that were made in the 1960/70's and fooled all the auction house for years but he does say that the Romanian and Czech copies are technically equal to the originals.
Specifically on Daum, he says they were the only cameo to use multicoloured bodies but is generally so well blended that there are no clear boundaries. The repros have very clearly defined spatter and mottling.