I've been asked once or twice, but usually I'm not asked. The last time I was there they were having a big argument with some guy who claimed he was an AMEX Plat holder but didn't bring his card, and he wanted them to talk to someone in his office who could read them his CCN as proof! 2 mins later some gal tried to just breeze past the check in desk, but they tackled her or something. I think there's more danger of that than there is of people stealing wallets or lending membership cards just to get into the Admiral's Club.