Engine jake brake how do I know it is working

I've tried the dealership but they know about as much as l do. I can only guess they must not sell many super duty trucks. 2019 ford 350

The factory optional engine brake, as equipped, can be turned to the following functional positions: On, Off and Auto. This restricts the Diesel engine's turbocharger exhaust flow, creating exhaust back pressure which slows the vehicle before even the engagement of brakes or downshifting of the transmission. You will know it is working when you feel the resistance kick in once the load and grade is of a significant enough amount. Otherwise, it will still work, but it's variable in affect felt due to the natural inertia of an unladen truck verses that of one towing the fifth wheel.