Proof mark.
This is a hot topic of mine right now. Dingus pretty well nailed the answer. If it has the 'fish' proof mark (oval surmounted with a crown) it is probably a no go for antique status.
Pre 1898 proof was the ELG in oval, no crown. The crown oval was brought into use in 1893 for arms specifically for export to Germany. However, this same proof later became standard and was applied to all arms as of 1898.
So if you can prove that this unit was made for German export pre 1898, (serial number relate to a specific contract, good luck with that one) then you can prove it as antique.
Proof mark is usually marked on face of cylinder, however, many pistols' cylinders are shaved to allow for use of 45 acp brass with moon clips, in such case they show no proof mark, pre or post 1898, which would make it impossible to date either way. Then you would have to find documented evidence, or another example of same make pistol that has antique status as a reference.