To be "positive" about your material supply, next time make sure you have a procedure to control/document the raw material stock thru purchasing/receiving/inventory/fabrication/acceptance/etc. If the sheet material in question is not a significant cost, it would be best to order new material with documentation that can be used to positively track the material throughout your manufacturing process.
Identifying rubber is an art. You can burn a piece and see and smell it and differentiate between them. I used to work in for Military & Aerospace company where there was a lab and experts that by very simple techniques could tell the difference, Sure, there will be cases that more accurate scientific methods are needed. I suggest you google for such techniques.