If you're doing it repeatedly, they will catch you. Modern data analytics are pretty powerful.

If you're thinking about doing it once, just buy a damn daypass. Lounge access in 2016 is cheap unless you're talking about the best international F lounges. And if you're willing to tie up thousands of dollars on a credit card to get into one of those, then I don't have an answer for you. I personally think it's unethical. I'd feel like a total d-bag doing it.