With a compression tester. It's a device that fits into the spark plug hole. You crank the engine and read compression in a gauge.

Typically you cut power to the fuel pump, remove all spark plugs, wedge the throttle full open and fit the tester to each cylinder one-by-one noting the max pressure reading after about 10 full cranks. There's a rhythmic sound to cranking. Count 10 cycles of the rhythm.

The easier gauges to use thread into the spark plug hole and have a peak hold feature. Don't forget to release pressure before removing the gauge. Other, usually cheaper, gauges have to be held against the spark plug hole and read while cranking. When cranking stops the gauge goes to zero.

Sixto
95 S420
87 300SDL