There are many ways to determine on which chain your POAP resides. The easiest method is by heading to the POAP Scanner and connecting your wallet, or inputting your Ethereum address/ENS, to view your collection.
When your collection finishes loading, click on the POAP in question and scroll down to the bottom of the page. If you see the "Migrate POAP" button it means that the POAP resides on Gnosis Chain (previously xDai). If you do not see the “Migrate POAP” button, it means that your POAP is on Mainnet.