Is there a way to measure U-values on older windows to determine if replacement makes sense?
There are already some leaks in the 2x panes (moisture), but they were expensive windows when installed in 1994. They are Marvin wood-clad.
You can get a relatively cheap thermal gun and determine the temperatures the windows are holding. You can also look up the window U-value and give it a rating factor based on age and observed performance.
Overall, it is going to be a ballpark educated calculation either based on thermal reading or calculated published values.
Depending on the glass-to-floor-area ratio, cost of utilities in your area, incentives available, and actual ability to improve performance, a window replacement could have anywhere from a 10 to 50-year ROI.
For more information:
Read Greg Upwall's Q&A "Moisture is leaking through our new basement windows. What should we do?"
Also, read Daniel Glickman's Q&A "We need to replace the windows on our home. What's the greenest choice?"