how can we know when our computer was last opened, if history is deleted?

Answer 1

Find Event Manager on your computer. Depending on which version of Windows, it is either in the Accessories >> System path or the Settings >> Control Panel >> Administrative Tools path. If you open "system events" you can see if anyone logged in. If you don't have your system set up for logins, you should still see some activity associated with a session at a time when you weren't there. Warning - it DOES take a bit of time to read through the system event logs.

Answer 2

Go to Recent Documents.