Create a calculation field - NotesExist - that has a number result:
If ( IsEmpty ( Notes ); 0; 1 )
Place the field on the layout where you need it and format it as a checkbox using a value list with a single value - 1. Resize the field so you can see only the checkbox and not the number.
Users will not be able to use the checkbox since it is checked by calculation. So it is probably best to adjust the field behaviour so it is not accessible in Browse mode. But it would be useful to be accessible in Find mode.
See how that works for you.
It's perfect! Thank you so much.
Just a note, for those who may read this. In this particular layout, Address, Notes, and Related Contacts are on different tabs. When I added the checkbox, it only appeared on the currently selected tab, so I had to copy and paste it onto the other two in order to see it checked when in any of the other two.
Hello - this is a thank you for 'expounding' on your solution. So many times I see a question answered or asked and not replied with details about how the OP (you in this case) used the solution. Sometimes the OP will reply to their own question - "Oh nevermind, I figured it out" - these turn up in my searches for solutions and .... grrrrr! ;)
Kudos! to you.