There are two 8-hour exams you have to take to be able to legally call yourself a Professional Engineer. The first is called the Fundamentals of Engineering (FE) and it's the same for all branches - you have to answer questions related to basic things like strength of materials, AC & DC circuits, fluid dynamics and so on.
The second is the actual Professional Engineering (PE) exam. This is the one specific to your chosen field, with only a couple of topics common to them all, like ethics and engineering economics. In many states, you are not allowed to take the PE exam until 4 years after you passed the FE exam. During that period, your work has to be supervised by a Professional Engineer, and your supervisor and four other people have to attest to your work when you apply to take the test.
After you pass the exam, you are allowed to buy a crimp-seal and a rubber stamp. The seal is round so it can serve as a bulls-eye when your clients aims are turned to litigation.