Then you either log in online, or go to an Oyster card machine (in every tube and London train station) to check.

Go into any shop that does oyster, they can put it on the reader, and the credit comes up, or ask the bus driver next time you use it.

Go into a reg varney shop and ask them to give you the balance.

You can do it online or use one of the ticket machines in a tube station

http://www.tfl.gov.uk/fares-and-payments...