Here is an indicator for checking the belt tension.
As I said, this is an indicator. It does not measure belt tension, it only indicates it roughly.
It can be used to make sure that all belts are tensioned about the same.
Simply make sure that the needle is in the same position on each belt and you know that the tension is the same on each belt. You won't know if it is tight enough, only that they are the same.
I have included the Step file, so if the indicator is too weak, you can easily stiffen the indicator by increasing the size of the "spring" between the main body and the middle post.
If you have any suggestions or improvements to pass along, I'd be happy to implement them.