Question
Is it possible to check what promotion was applied to subscription?
Answer
It is not possible to find this information in control panel.
You can find in PBA-E database in table Subscription:
pba=> select "subscriptionID","PromoID","PromoCode" from "Subscription" where "subscriptionID" = 10000001;
subscriptionID | PromoID | PromoCode
----------------+-----------+-----------
<subID> | <PromoID> | PROMOCODE
(1 row)