Spotting fakes in general is usually quite easy and most of my suggestions will apply across a wide range of goods.

The first clue is the packaging, fake is generally good but that of B&O will be perfect ie. the silver lettering will be perfectly parallel with the edge of the box.

Build quality, there will be no rough edges on any quality goods in particular feel around the edge of the earpieces.

There should also be no discolouration on any part of the headphones.

The stitching around the edge of the leather pouch should be perfect and the same distance from the edge with no variation at all.

The leather used and any other materials will be of the highest quality on genuine goods.

There should of course be a genuine and usable guarantee card.

The final point is that as you are paying for quality they should sound superb.