How to judge whether the O-ring is scrapped?

After a period of use, the O-ring will age, and it needs to be replaced at this time. How to identify whether the O-ring can still be used?

1 When the O-ring occurs, the O-ring becomes brittle as a whole, and irregular cracks appear on the surface. At this time, it means that the O-ring is seriously aged and needs to be replaced immediately.
2 It should be that the O-ring is made of rubber and other products, and has a certain elasticity. We can stretch it by hand to see. If we find that the O-ring cannot be recovered immediately after being stretched, it means that the O-ring is not good. to be replaced.
3 When white spots appear on the surface of the O-ring, or stickiness occurs when you wipe it with your hands, it means that the O-ring is oxidized and cannot be used further.