You can try any of the following.
Download an app for a smart phone that will allow you to check the rpm of the engine using the torch as a strobe. I tried this and it was a right PITA. So gave up.
Use a strobe to check it. Have not got one so did not do this.
Alternatively , to double check mine, I used the ashcroft ratio calculator to determine the RPM at a given speed ie 60 MPH and what the RPM should be for my specific tyre
size etc and made sure it was correct.
Cheers