My default move is to just add sealant, because who's got time for testing?
Shaking might hint at sealant levels but usually not unless they're big tyres with a lot of sealant. If I really wanted to know I'd pull the tyre off and eyeball it, but I'm more likely to pull the valve out and stick something in to act like a dipstick to check fluid level in the tyre.
I have a very thin zip tie i've kept hold of for precisely this purpose, actually gives a decent indication of whether the sealant is still sealant, rather than separated out eggwhite grossness
My default move is to just add sealant, because who's got time for testing?
Shaking might hint at sealant levels but usually not unless they're big tyres with a lot of sealant. If I really wanted to know I'd pull the tyre off and eyeball it, but I'm more likely to pull the valve out and stick something in to act like a dipstick to check fluid level in the tyre.