How can an alien claim to be a US citizen? Typically, an illegal immigrant will check the box on an I-9 form in order to work without having any other papers. This is the most serious form of the crime along with filing for welfare and other such financial benefits. A few aliens will sometimes check the box and register to vote at the DMV while getting a driver's license. There have also been cases where some foreigners who live legally did not realize that they cannot vote and have deliberately registered to vote and have even voted in elections.
How does USCIS deal with immigrants who have claimed US citizenship? Typically, an alien who has evern claimed to be an American will not be able to get a green card and immediate deportation is issued. It is not clear what law Congress will pass as part of Obama Amnesty and if it will specifically condone such action, but it seems highly unlikely.
How does USCIS find out that an alien has claimed to be a US citizen? When an alien will file Form I-485 for adjustment of status (a standard form to be submitted to apply for permanent residency in the USA) -- this is the form that all undocumented immigrants will also follow to apply for green cards -- the USCIS will look very carefully at your application. It is is not clear if all illegal immigrants will need to appear for an interview, if the officer has any doubt that an you may have claimed to be a US citizen, you maybe issued an RFE or ordered to appear for an interview. USCIS will subpoena the current and previous employers to send all I-9 forms that you submitted. Most employers simply mail a copy of your employee file. So when you go for the interview, you are not aware of what exactly the adjudication officer knows about you (there will be a lot of information through your background check), you have to take an oath to tell the truth (remember that the interview is a legal proceeding and you cannot commit perjury), and then you will be order to answer how you got a job without authorization. If you knowingly claimed US citizenship when you knew you were not a citizen, the officer will refuse to approve your application, put a permanent bar on entry into the United States, and start removal proceedings.
What are your options if you are in this situation? It is probable that Obama amnesty will give you a provisional legal status (right to live, travel, and work) without asking you if you ever claimed US citizenship. In that case, while you may never be able to apply for a green card, you will be able to live in America in the provisional status.
What if you claimed to be a US citizen because you were made to believe that you were a citizen? If you can show a judge that you genuinely believed that you were a citizen when you checked the US citizen box on an I-9 form, you will be fine, but this is not going to be easy.
What should I do if I claimed US citizenship? Do it yourself application will not work for you. You must consult with an excellent lawyer because it might be better for you not to apply and face deportation or maybe you had a genuine situation in which you did not intentionally lie.