Help to understand 'log space used' metric
You can check your backup history by using a standard report:



... by using some scripts:
Script to retrieve SQL Server database backup history and no backups

--------------------------------------------------------------------------------- 
--Database Backups for all databases For Previous Week 
--------------------------------------------------------------------------------- 
SELECT 
CONVERT(CHAR(100), SERVERPROPERTY('Servername')) AS Server, 
msdb.dbo.backupset.database_name, 
msdb.dbo.backupset.backup_start_date, 
msdb.dbo.backupset.backup_finish_date, 
msdb.dbo.backupset.expiration_date, 
CASE msdb..backupset.type 
WHEN 'D' THEN 'Database' 
WHEN 'L' THEN 'Log' 
END AS backup_type, 
msdb.dbo.backupset.backup_size, 
msdb.dbo.backupmediafamily.logical_device_name, 
msdb.dbo.backupmediafamily.physical_device_name, 
msdb.dbo.backupset.name AS backupset_name, 
msdb.dbo.backupset.description 
FROM msdb.dbo.backupmediafamily 
INNER JOIN msdb.dbo.backupset ON msdb.dbo.backupmediafamily.media_set_id = msdb.dbo.backupset.media_set_id 
WHERE (CONVERT(datetime, msdb.dbo.backupset.backup_start_date, 102) &gt;= GETDATE() - 7) 
ORDER BY 
msdb.dbo.backupset.database_name, 
msdb.dbo.backupset.backup_finish_date 


... or having a look to your server's ERRORLOG file and searching for Source = Backup