Hi Craig,
Select that cell, go to Data --> Validation. Select Custom in the top drop down box and use a formula like this ...
=A1<=16
Assuming that you are in cell A1; change as needed. You can set custom popup messages if you'd like. You can 'mark' it with Conditional Formatting if that would be easier. That would be doing things like changing the font or background color of it when such a condition exists (that you specify), but with no popup.