It depends how the family is modeled. A lot of families have a parameter called exterior offset or something like that, so the easiest would be to add this to the family if it’s not already there, no Dynamo needed. In your screenshot it seems you have two exterior planes of the glass so it would be even harder to decide which plane you want to measure from. I would modify the family with like 3 clicks, instead of trying to reinvent the wheel in Dynamo.
If you want to do this with dynamo, and the family has correct subcategories assigned (so this workflow also depends on how the family is modeled), you can select the face of the wall and the face of the Glass subcategory, and calculate the distance between them.
About filtering for subcategory read this thread, or search for ‘subcategory’, there are some threads about this topic