Check if backup is in progress

is there any way how to check that postgres 9.1 is currently in backup mode? 

Imagine the situation that my DB is running and the backup is remotely started by 

# select pg_start_backup('backup', true);

However the remote site crashed and can't stop the backup. Is there any way how to check if something like it happen or somehow configure the timeout for backup procedure? 

Currently only solution that comes to my mind is to try to start another backup, if this fails, I know that it is in progress.

Thanks

Stano