Spectrogram is a Windows-based audio spectrum analyzer that uses the computer sound card. Originally freeware, later versions had more features and required a registration fee to be paid. I first came across the program when building my Elecraft K2 back in 1999. By looking at the spectrum of band noise, you could easily set up the crystal filters so that both sides are evenly balanced and the passband where you want it to be.
The company that developed Spectrogram, Visualization Software LLC, is closing down at the end of May, and according to a notice on the download page, the website and downloads will not be available after that. The last version, Spectrogram 16, is now freeware again. So grab it while you can. You never know when you might need it.