There are two generic ways to measure the speed. One is to use an ordinary automotive servicing tachometer with a plug-lead connection (unless it's a ride-on mower you'll need a 12 volt battery handy) and the other is to use a stroboscope. Many mowers - especially older Briggs engines - have a visible rotating part on top, so it's easy: you just put a bit of sticky tape on that temporarily, and you are good to go. If there is no rotating part on top it is very, very difficult to use a stroboscope safely. In theory you could remove all of the blades, put the four wheels on solid blocks or up on the bench, chock the wheels, start the engine, and point the stroboscope at the blade disk. I do not recommend that approach because if the mower moves - for example, when starting it - horrific injury could happen to someone even with the blades removed. Using a plug lead tachometer is the only sensible, professional way to do the job. And never, never lift a mower while it is running. Starting it while it is up on a bench is risky, but lifting a running mower is just insane, even with the blades removed.
