The Multi-Engine Rating can be added to just about any existing airman certificate. Additionally, an initial checkride can be conducted in the multi-engine airplane; for example, taking a Commercial Pilot checkride in a twin-engine airplane. The areas that will be tested, however, depend on what the rating is added to for the pilot certificate. The requirements for testing can all be found in the respective ACS or PTS. For example, if wanting to add multi-engine privileges to a Private Pilot certificate, go to the Private Pilot ACS and review that section. For Commercial privileges, visit the Commercial ACS.
Regardless, the multi-engine rating is a great addition and opens the doors to many flying opportunities! Please contact us if you have specific questions about this exam. As always, due to the complexity of multi-engine aircraft, certain DPE's in your area may only able to conduct this checkride in certain types of twin-engine airplanes.