Currently, you can only check it at your customer portal:
- my.tokyotreat.com/credit/referrals for TokyoTreat, or
- my.yumetwins.com/credit/referrals for YumeTwins, or
- my.nomakenolife.com/credit/referrals for nomakenolife
However, we will keep improving the feature to inform you when your friend already signed up. Just make sure your friend subscribed using your referral link to get discount, and you will earn the credit!