At times, I am asked by potential clients with overstay issues how USCIS could possibly find out that they have been in the US longer than their permitted time. Sometimes, the question is “how will USCIS know that my marriage isn’t a real one?”
It’s never advisable to lie on an immigration application, as this couple learned. In this case, they got pretty far, too… a Tunisian man got US citizenship through marriage to a US Citizen, then got his new wife a green card; his US Citizen wife who originally petitioned him married another foreigner and got him his permanent residence. The Tunisian is being deported after serving time in jail; his “wife” who petitioned him is facing up to 21 months in the federal penitentiary.
My answer is generally that “they” will find out. Every additional lie complicates the case, making it easier for a sharp adjudications officer to see inconsistencies and discover the fraud.