How do you know that you are illegally staying in the United States?

You can check it through the US Immigration Service. In the era of big data, everything you want to know is in your computer.

According to the immigration nationality law of the United States, overstaying in the United States will be punished by three levels:

If you stay illegally for more than 180 days but less than one year, but leave before any formal repatriation procedure (deportation), you will be banned from entering the United States for three years.

If you stay illegally for more than one year (365 days), but leave before any formal repatriation procedure (deportation), you will be banned from entering the United States for ten years.

If the cumulative illegal residence exceeds one year (cumulative time, not necessarily continuous), or if you try to enter the United States without border inspection after being deported, you will be permanently banned from entering the United States, and there is no immunity except VAWA private prosecutor.