The other day I found some oxygen absorbers that had been shoved to the back of a cupboard and forgotten. I think that they were about 3 or 4 years old. They were sealed only with the temporary clips that are normally used for short periods during canning operations. Being curious I decided to test them.
The normal way you test them is to tape them to the inside bottom of a quart jar. The jar is then turned upside down in a pan of water. Over the next 24 hours, the vacuum they create should cause the water to rise approximately 2 inch up into the jar.
As you can see from the pictures, these failed miserably. Oxygen absorbers normally contain powdered iron and a salt. The oxygen is used up when the iron oxidizes or rusts. I took a couple of the failed ones and cut them open. You will notice in the picture below that the iron has turned to reddish brown rust. In a new absorber the iron will bright and shiny.