After the Layer 2 solution is released, if you have already minted domains and are wondering how to determine whether the domain is currently on Layer 1 (Ethereum) or Layer 2 (Polygon), there are a couple of methods you may use to identify.
Solution (A)
You can view the icon next to your minted domains from the My Domains page.
Polygon
Ethereum
Solution (B)
Connect your wallet to Opensea.io and go to Profile.
Click the domain in question.
If the domain is on Layer 1, the URL will begin with: https://opensea.io/assets/
If the domain is on Layer 2, the URL will begin with: https://opensea.io/assets/matic/