If you've registered for an Oyster card account you can log in and check your balance online.
If you've not yet got an account you can still do so (although, annoyingly, the registration facility is down for maintenance at the moment):
https://tfl.gov.uk/fares-and-payments/oyster/oyster-online-account#on-this-page-1
Alternatively you can go to any London Underground station and simply tap your card against the card reader on a ticket machine. That will display the balance.
Or, as MallyJ says, go to any shop that sells top-ups (which is almost every convenience store in London) and get them to check the balance for you.