Hi there! The following should answer your question.:
The power of the President is checked in the United States is checked by the Judicial Branch and the Legislative Branch. The Legislative Branch checks the president by signing any treaties the president wants to put in place and being able to override vetoes. The Judicial Branch checks the president by declaring any if his/her acts unconstitutional. The systems put into place to ensure that presidential power is not unlimited is called Separation of Powers!
Under no circumstances should presidential power be unchecked, for that would be against the ideas presented in the Declaration of Independence!