Pull on the nylon webbing and stitching, if it tears or the stitching breaks…the vest should be replaced.
If the work-vest has damage to the flotation then it should be replaced.
If the flotation looks as though it is “flat” and has lost buoyancy, replace it.
If the work-vest shows excessive wear, replace it.
Keeping the work-vests out of the sun when not being used will help them last and perform better than constant exposure to UV.