Practice, practice, practice.
Many modes can be identified by ear. Of course, the problem is you have to know what they sound like to ID them by ear. Audio recordings can help you to learn what some modes sound like. The recordings linked in the MultiPSK thread are great, but mostly only covers things to be found on the ham bands.
Analysis by ear can only tell you so much, and is prone to error with some modes. So tools are also good to have in the collection. Programs such as Spectrogram, SpectrumLab, Argo, Audacity, etc, will help you take apart and “see” the waveforms features.
There are specific analysis tools that are designed for taking these signals apart, decoding (as long as not encrypted), and even automatically identifying them. For the most part these tools are not free and some are very expensive for the average hobbyist. Such tools include Hoka Code300, Krypto500, Sorcerer, etc. The first 2 cost thousands of dollars US per copy, Sorcerer can be found on the web for free although I am not sure the legal status of such copies.
There are a few web pages with examples of digital signals on them, both audio and images of the waveforms:
Digital Mode Samples » K3DCW - Amateur Radio - Real radio bounces off the sky
Sound samples examples digi modes
(mostly ham, but a few others)
The first and last are the best resources I have found. The last one listed does have some pretty heavy anti-lurker features to it, and you may have to email the owner to be allowed access to the web site. The data there is worth seeing.
Don’t forget that YouTube can be your friend to ID some SW transmissions. Although it can be tough to sort out the wheat from the chaff there are lots of good recordings of modes on YT. I am trying to expand the digital examples on my YT channel, although right now the digital modes are secondary in my postings, numbers, oddities, and pirates are primary. My channel here: FirstToken's channel - YouTube
If utilities is a thing you are really interested (Utilities, or Utes, are pretty much any SW transmission that is not Ham or Broadcast) you might look into the #wunclub chat room. The #wunclub channel can be accessed via Java Web chat at JavaChat
. If you have or use an IRC client it is on the Starchat sever.
The #wunclub is a very long running gathering location for Utility monitors, it has been active in one form or another for many years. It is a site/community where you can throw a frequency into the chat and get other peoples opinions as to what the signal is, and possibly its general location based on who has it strong, weak, and not at all. Several of the users have been doing this for many years, some professionally or have done such work professionally in the past. There is also a logbot that can be used to look up past reportings, and add your new intercepts.
This is a place you can ask in real time “what is that digital mode on XXXXX kHz” and possibly get an answer. Learning the modes as you go.
Keep in mind that if you do go in there and no one answers your question, despite there being a couple dozen names in the room, it does not mean anyone is ignoring you. Many of us stay logged into the room 24 hours a day, to grab any frequencies thrown into the chat when we are not present. Some users are active in the European morning and afternoon slots, some in the US morning and afternoon, and others in the Pacific time slots, so you might be there when everyone else is off at work/dinner/sleeping, etc. If no one responds either stay in the room or come back in a few hours and try again, you will eventually figure out who is active when.
Because there are some pretty inane conspiracy types who come into that room from time to time (generally in a surge for a day or two after some web blog or article is posted that list the site and its activities) some of the people in the rooms initial responses to new users can seem a bit caustic, particularly if the new person comes in with some obvious newb or inciting name (example, with the default “Guest” name of the Java client). If you choose to visit the #wunclub come on in, introduce yourself, and ask questions.