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?"