Well, 40+ years of listening for me. I used to heavily rely on my monthly issue of "Monitoring Times" way back when but now rely on what others post and some of the blogs on the Internet. I see an interesting post and I go listen and learn.
The Signal Identification Guide at the Sig ID Wiki is good:
https://www.sigidwiki.com/wiki/Signal_Identification_GuideAs for my post with "KG-84C SYNC with 100% confidence" that is purely experience. Encrypted RTTY has a certain cadence to it. The "sync" is transmitted with a series of RYRYRYRYRY. It becomes obvious when you listen to it for long enough.
As for the actual decode, I generally use Sorcerer. I will also use multi-psk or fldigi and just sit there and transmit in all of the various modes and learn what they sound like. There are some real gurus out there and like I, they hang out at the UDXF groups.io.
In my opinion, Antonio happens to be one of the best, and very technical in nature, and you can find his extensive blog at:
http://i56578-swl.blogspot.com/