I was asked how to tell if a lid has been used. You can tell by the sealing compound. If it is used it will show an indentation from the rim of the jar.
If not used... it will be smooth. I took a picture of two lids. One is used - One is new. Can you tell the difference? (answer below the picture... don't cheat! Decide first then scroll down and look.)
It might be hard to tell but the lid on top with the gold finish is the new the lid. The lid underneath with the white finish is the used lid. Another way to put it, the lid on the right is the new the lid on the left is the used.
This of course leads to the next question... does it matter. My answer is Yes, it does matter.
This indent in the sealing compound makes a failed seal more likely if you try to reuse the lid. I don't like to waste my time and food for a higher chance of failure. So my recommendation is... don't reuse lids. (It also happens to be the recommendation of USDA and all traditional lid manufacturers that I know of.)
I just know that I will get tons of replies from folks who have reused lids. I don't recommend it.. you have to make your own decision. Remember there are no canning police. :)
One option you might consider...
Or.. Recycle those used lids.
Lids can be reused on jars for storing other items. I asked for and got some great ideas here.