use the backup details in msdb - this query will tell you when a database was last backed up (full backup)

select a.name, b.backup_finish_date, a.crdate

from master..sysdatabases a left join msdb..backupset b on a.name = b.database_name

where b.backup_finish_date IS NULL or b.backup_finish_date = (select max(backup_finish_date)

from msdb..backupset where database_name = a.name and type = 'D')