Piers Morgan -- I've Known Lupita's Dress Was Fake Since Oscar Night

It didn't take a fashion expert to know Lupita Nyong'o's stolen "pearl" dress was fake -- since Piers Morgan claims he knew it was bogus the second he saw it on Oscar night.

We got Morgan in NYC Monday ... and he says he rubbed elbows with the Oscar winner at the Vanity Fair party, and was close enough to realize the gown was all smoke and mirrors. Watch him explain why it fooled people without his keen eye.

As for the theft of the dress originally believed to be worth over $150K ... well, he only sees one loser in all of this.

We'll hand it to Piers ... his take on the heist is entertaining, but it might be slightly off -- since it turns out Lupita attended the VF bash in a different gown ... a slinky gold number.

Who knows? Maybe this one was really blue and black.