Assume you have a colour mix node that you are using to determine the roughness value of a Glossy BSDF. Is there any way to actually see the roughness value? Is there a way to see set values of nodes in general, if they are not set "in place"?
Plainly as a number, no. Approximately, yes.
You can plug anything into the color input of an emission node and plug that into the surface material output. This will let you visualize the values across the mesh in the viewport with rendered viewport mode.
There is a handy addon written by Greg Zaal, called node wrangler, which (along with a billion other useful things) will do this automatically by Ctrl + Shift + LMB (left clicking) on a node.
To make numerical values easier to distinguish you can plug the value into a color ramp node first, then you can create a colorful gradient in the color ramp which will make it easier to see the value in the viewport.