How to make measurement units visible permanently or in on/off mode?

More information please.

Are you referring to dimensions placed with the Dimension tool? If so, those dimensions can be placed on a layer that is set to be either visible or invisible for your scene as desired.

More info needed…

But if you did mean the ‘units suffix’, then it’s a checkbox in the Model Info > Units dialog.
So ON >>> 123.4 cm, OFF >>> 123.4
Also note that in the Model Info > Dimensions > Expert dimensions setting dialog, you can change the foreshortening angle at which dimensions are hidden, or stop this hiding completely - as you also can for hiding ‘small’ dimensions…