Pallets can become contaminated with chemicals spilled on them during the shipping process. If there are any spills on it, either oil, food or unknown substances, do not use this pallet! Pallets are frequently used to transport dangerous or toxic chemicals/liquids. It is much safer to use only clean ones and not try to identify what might be on your pallet.
Once you have found a clean pallet, the next step is to check for a stamp or marking on the sides of the pallet.
You can find more information on pallet safety in this article we’ve made: Is my pallet safe for reuse?