The trick is that no one knows it is a test. That is not announced.
For starters, there is willingness to trust.
For the thoughtful, that I'm not about to saddle some one with a guy who probably won't be compatible. Even if she failed, I'd finish the date, and have fun, but in all likelyhood turn down any offer to spend the night. No point in striking sparks where you don't want a fire.
Keep in mind, too, that when the test was devised, a $20 was a lot more money than it is now. In those days it would buy a couple of cases of beer or a really good steak dinner for two.
Tricks are for kids... Order your own drink and be honest with people who you want to be honest with you.
Here in L.A., if you did that, you'd be the one failing the test, since a 20 wouldn't cover two cocktails plus tip at anyplace reasonably nice. You'd have shown yourself to be a cheapskate.