How can you find out the uplay account which is linked to Steam?
No there is no way to find out which Uplay account is linked to which game.
If you have multiple accounts of Uplay then you have to be careful about which account is signed in. Steam clearly gives you keys which you have to activate on Uplay client.
As for your case, you must remember the email ID of the Uplay account otherwise, I am afraid even Uplay Customer support won't be able to help you.