Shine a light through the lens, look for dust, also look for spiderweb looking stuff (could be fungus) inside the lenses.
look in the lens, close and open the aperture and look for oil on the blades.
Feel the foam sealing if there is any and if it's sticky and peeling / flaking off.
Listen to the shutter as it clicks, they should both sound similar-ish
Other than that, run some film through them and check the accuracy of the shutter.
You'll need a meter to get the correct settings.
The Noteworthy Ones - Mamiya: 7 II, RZ67 Pro II / Canon: 1V, AE-1 / Kodak: No 1 Pocket Autographic, No 1A Pocket Autographic
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