I think all airlines are different when it comes to checking status.
When I reached *G it took a while before my card arrived, it got lost in the mail.
During my first trip some lounges accepted the BP as a proof straight away, for example in the SAS lounges they just swipe the BP and get the reservation on their screen where they can see the card is valid.
In other lounges not even *G on my BP + a printout of my account status (where it clearly said level: gold) made it, but I was persistent and asked the lounge assistant to call SAS to verify that the card was valid. They weren't too happy about it (would have been a lot easier to just ask me to leave) but they did it and I was allowed to enter.
So some airlines check your status very carefully (even wanting to see the expiry date on the card) while other spend about a second looking at the BP before letting you in.

You can always call the FF-helpline and ask them to fax a paper confirming your status which you can show if they ask for your card.