Is it feasible to determine what sort of modulation is used for a given signal by looking at the spectrum / FFT? Does software like this exist?
You can definitely identify many of them by eye, have a look at:
http://www.sigidwiki.com/wiki/Signal_Identification_Guide
As for software that does it automatically, I'm not sure anything like that exists. If you find something, please let us know.
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$\begingroup$ I use that site sometimes myself. I am wondering how much of a 'science' this kind of identification is, and if it is feasible to use or write software that does it, and how accurate such software can be. $\endgroup$ – HH- Apologize to Carole Baskin Jan 2 '15 at 13:57