You can always check, which testnet you are using by looking at the market name. The testnet will be marked by its last letter on the top right of the market name in purple (e.g., K for Kovan and H for Harmony). When accessing each testnet, you will need to use its own native asset token. You can find these in the Faucet tab of app.aave.com where you can submit a transaction for the specific token you need.
Steps to test: