how do I check when my last backup was on iCloud drive

Apple offers iCloud Backup for iPhones and iPads but not for Mac computers.

iCloud Drive is a syncing service and automatically syncs individual files as changes occur. You can open a Finder window and go to iCloud Drive folder by using the Finder sidebar or the Finder "Go" menu. In Finder you can view the current status of syncing by looking at the icons in the column that has a cloud icon for the column header.

More information that may help:

It is possible to sync data from a Mac to iCloud if you turn on those options (e.g., iCloud Photos, iCloud Drive including the Desktop & Documents option). Syncing has some of the benefits of a backup (and the benefit of being offsite) but is not the same as a backup. Major differences include the fact that not everything on your Mac can be synced and the fact that if you delete something, accidentally or otherwise, there is only a limited time period (30 days in most cases) during which it can be recovered.

Also see: https://support.apple.com/mac-backup

And more information on syncing in:

https://support.apple.com/en-us/HT204264

and

https://support.apple.com/en-us/HT201104

If you had Time Machine turned on and you always or occasionally connected an external drive for it to backup to, you should have that as a backup; but Time Machine will not backup to iCloud.

For your wife's MacBook Air you may want to use an internet browser and log into www.icloud.com using her Apple ID and see if there is any of here data there.

For your 2012, if you can't resolve the external storage issue, you may want to try syncing as much as you can to iCloud and also to a 3rd party cloud backup service