It is very easy:

1) Place plaque on a flat piece of glass.

(i) if you can see under it, it has a bow, & is not flat.

(ii) if it wobbles on the glass, it is warped

2) place a popsicle stick over the lunette, then place a tooth pick perpendicular to the stick, & into the lunette recess Then put a mark on the tooth pick at the intersection of the popsicle stick. Then just measure the distance the tooth pick when into the lunette.

3) Shine a flashlight through the lunette, and check it for clarity, there should be no cracks. When the lunette drys out, it will resemble decomposing cellulose nitrate dice.

4) The smell of formaldehyde, is a warning the lunette is deteriorating. Formaldehyde is the preservative B et G used in the early plaques.