how to tell when backing up to time machine is done?

If TM is actively backing up, the menubar icon has two arrowheads:

The "Preparing Backup" status can last a long time if you've not backed up in a while. Likewise, if your backup drive is a slow USB1 or USB22 model, things will be slow.

VikingOSX wrote:

Allan,

Apple has redesigned the Time Machine icon to look like the following in macOS 11.6:

https://discussions.apple.com/content/attachment/109ff44c-dd80-4bab-947f-ecb34be7cc2f

and the best way to know when it has really finished is by clicking the preceding icon to see the following status report:

https://discussions.apple.com/content/attachment/51a92ccf-de9d-4f9c-91f8-f8405d142505

Out of town, so could not do these captures from my 2013 iMac still running Mojave.

Yes, I know. OP does appears to be running Mojave:

Out of town, so could not do these captures from my 2013 iMac still running Mojave.

Ain't that a pain! I keep a lot of my oft-used screenies in Dropbox, but too often I don't have the right ones when on the road!

Allan,

Apple has redesigned the Time Machine icon to look like the following in macOS 11.6:

and the best way to know when it has really finished is by clicking the preceding icon to see the following status report:

Out of town, so could not do these captures from my 2013 iMac still running Mojave.

I haven't done it for screenshots, but all of my various scripts and compilable code is stored in titular directories on iCloud in an outer Code folder. So, when on the road, I have access to that Code folder via iCloud Drive in the Finder sidebar and in the Terminal application via:

mkdir -p ~/iDrive ~/Code
ln -s /Users/$USER/Library/Mobile\ Documents/com~apple~CloudDocs /Users/$USER/iDrive
ln -s /Users/$USER/Library/Mobile\ Documents/com~apple~CloudDocs/Code /Users/$USER/Code

and both of these iCloud Drive folders show up locally on my Mac as:

You can tell by looking at the icon. If Time Machine is backing up the icon changes. Or, by clicking on the icon and checking the status.