One method I've used is to either remove the shock from the vehicle or just get one end loose and then try to rapidly compress and extend it by hand to check the resistance. A lot of shocks will have minimal resistance in jounce (pushing the shock together) even when new, but when you get little to no resistance in either direction, it's time for new shocks.

Of course, when the shock pukes shock oil everywhere, that's a pretty good clue that it's done

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.