If you can't find a label certificate
Most energy codes require NFRC ratings or direct manufacturers to use punitive default values found in the relevant energy code. If the window products you are inspecting have no rating, you may assign it the default value based on your state’s energy code default. For any version of ASHRAE 90.1(download here)defaults, go to Normative Appendix A, table A8.2 to determine the default value. For 2009 and 2012 IECC defaults, see tables 303.1.3(1), 303.1.3(2), and 303.1.3(3) on the righthand column of this web page. For 2006 IECC defaults, see tables 102.1.3(1), 102.1.3(2), 103.1.3(3).
One other possibility
You can also contact NFRC to see if the window manufacturer may be able to generate a Label Certificate. If the manufacturer has not participated with NFRC before, encourage them to begin using NFRC ratings to avoid punitive defaults.