barrybritcher asked, "what's the test to know when these are shot?".
That question includes when they're near shot. Those damn bearing plates-automotive lazy susans-must be subject to variations in material quality like no other. There've been pictures of horridly rusty ones posted here. At 150K both of mine were new looking but for one thumb print of rust.
The test would have to be rust in the raceways and on the ball bearings themselves. Inspection would be difficult as there's precious little room for spraying lube into them. A friend had a tenacious, clear viscous lube in a spray can. The spray can's straw worked well enough. Alas, that particular product is no longer around.
So, if they (the bearing assemblies) look OK and roll well, you can lube 'em as best you can and reinstall. If they're rough running there's little to think about.
If they're rolling OK and have rust on their plates, you gotta make your own mind up I suppose. They're not too dear.
I'd see to the lubing of the balls and coat the entire thing with a hearty, outdoor worthy rust preventative.
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