Useful trick for canning
All the official guidance on canning disapproves the use of jars and lids not purpose designed for canning. Many, myself included, do use such jars and lids with success. Specifically, jars used for store bought canned items such as pickles, olives, etc. Invariably these have single piece screw on lids. This presents a bit of a problem in assessing whether a proper seal has been achieved. Normal two piece canning lids allows verification of sealing by removing the screw band and picking the jar up by the edges of the lid. Clearly this is not an option with a one piece lid. If the lid is sealed it will be somewhat concave due to the partial vacuum in the jar. If the lid is made from thicker than typical canning lid metal seeing the concavity may be a bit difficult without the trick I outline here. All that is required is a thin, short straight edged object such as a razor blade and a small flashlight. Using a razor blade, place it vertically with the edge centered on the jar to be tested. Shine the flashlight from behind the razor. If sealed and thus concave the razor will be touching the lid at it's ends only and the concave contour of the lid clearly visible. If not sealed the razor will be touching the lid all alone it's length. I recently did a batch of 20 such jars that originally held Wally World store bland sliced olives. Some of them were on their second reuse. I had only one seal failure out of 20. Photo below of a properly sealed lid.