A mechanical shutter is a small machine of springs and gears, and like any machine it tells you how it’s doing if you know how to listen.
You don’t need a shutter tester to catch the common faults. A quiet room and a bit of practice will get you most of the way.
Listen to the speeds
Fire the shutter through its range, from fast to slow. The fast speeds should sound crisp and more or less identical to one another. Then slow down. At 1/15, 1/8, and especially the full second, you want a smooth, even whir rather than a stutter or a hang. Slow speeds are the first thing to gum up as old lubricant stiffens, so that’s where trouble usually shows first.
Watch the curtains
On the slow speeds, open the back (no film in it) and fire toward a light. The curtains should travel evenly and the opening should look consistent across the frame. If it’s brighter on one side, a curtain is dragging. Check that neither curtain bounces or caps at the end of its travel.
Then the rest
After the shutter we work the film advance, the frame counter, the self-timer, and the meter if it has one, every mechanism, every time. Anything that lags gets written into the listing. A shutter that sounds right usually is, but we’d rather check than assume.