Mies van der Rohe - Barcelona Table - Copy or authentic?  

If its older you'd expect to ...
If its older you'd expect to see some fine scratches on the glass and perhaps some pitting if its chrome plated, which it should be. I do remember that the overall height should be more than 432mm, the 12mm thick steel is an interesting clue, if its of American manufacture it should be 12.7mm.
A book on Mies I've got says that the height should be 550mm and that Knoll produced it from '48 onwards and that they changed to stainless in '64.

Frame is polished and...
Frame is polished and non-magnetic (thus stainless steel?). How can I tell if it's chrome-plated?;
Frame is hollow section (does appear to be 12mm, but I haven't used callipers), with 5 holes in frame for plastic studs that sit on dome headed pins that locate in holes.
It could be that glass is replacement on original frame. Were different sized frames made pre-1947? What were the original frame plan dimensions?
Current Knoll model is 460mm height overall the glass. Glass is now 15mm, which with 5mm spacers would mean that current frame is 440mm high.

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I seriously doubt its what you want, I'd nab it for the glass if it was cheap but if a dealer is trying to pass it off tell them to stop lying to you.
I wonder when Knoll lowered it? Should be 1000x1000x550, originally came with clear, black glass or rosewood top, a dining version was offered by Bamberg and they may have offered it with red, yellow or blue lacquered steel. They'd be rare as hens teeth but just for extra information.