What did she order? Top shelf? Is that her usual? Is she a power drinker> did she get your order straight? what did she do with the change? did she tip? Your money or hers? You are learning a lot already...
Put the change on the table, and see if..she orders again and uses your money (asks, does not, pays for it herself and buys you one of what you are having?)...
You can eliminate a lot of non-starters for a twenty.
And what does that test say about you that is desirable to women?
If I were say.....a divorced woman out with this man that left the $20 and took off for the john, I'd be thinking,"Hell, I have done enough ordering and doing for the last one I was with, if bathroom boy can't wait to place an order, then to hell with this". Grab the $20 and run!
Clever move and though somewhat sneaky, it's fair.
Let's hear some other tips for easy information-gathering.