First, make sure your auction was settled (auctions only; if Buy Now, you can skip this).
Then, go to Etherscan, paste your wallet address into the search bar, and navigate to the Internal Txns tab located in the middle of the page.
There, you'll be able to see whether or not you have received payment. Make sure to scroll to the correct date and look for a Foundation: Market payment.
Please note that payments are not visible in the "Activity" tab in your wallet because that only shows transactions initiated by you.
