The only real way would be to open the compressor housing and measure. Per Modern Performance -
If the car was running 19 pounds of boost on factory sensors without a CMR tune, I wouldn't believe them and would walk.
Alternatively, you could check out the tune. My understanding is that the BWS could not run DiabloSport 93 octane level boost without a custom tune without running lean. If I saw 3BAR sensors and a custom tune, I'd likely trust the seller that there is a BWS. Especially if they had a receipt. Most people keep those for expensive items.
If the car was running 19 pounds of boost on factory sensors without a CMR tune, I wouldn't believe them and would walk.