If you spend any time listening in on the airwaves, you've probably noticed a lot of rather strange signals. If you followed some of the tutorials on this site, you also may have identified most of these signals.
You might be able to identify a Numbers Station during it's broadcast quite easily. Usually the station will repeat a series of numbers and letters or words, usually by voice, a few times. Even if you don't know foreign languages, just a voice droning in a halted pattern might be a giveaway that they are saying numbers and letters or words. Often the words would stand for letters of the alphabet (as in a phonetic or spelling alphabet) or just be code words for something. When these stations broadcast the information may be random or at certain times of the day or at certain intervals.
When they are not transmitting the numbers, there may be what is known as a channel marker being played. A channel marker is a pulse, tone, music or other sounds what basically keep the channel occupied with some sound so that others are less inclined to use that frequency to transmit. This in effect reserves that frequency for future use by the one that is transmitting the channel marker. The infamous UVB-76 is one such channel marker which has been dubbed the Russian Buzzer due to the pulsed buzzing tone that is continuously played when no numbers information is being broadcast. It would take some time and patience listening to a channel marker for any breaks which would indicate that numbers are about to be played. Some channel markers are designed to be rather stealth, posing as a normal radio station. Others play what some find outright creepy music or sounds. If you happen across any strange transmission, you might want to listen for awhile to see if it's a channel marker and if numbers will be coming up at some point.
Sometimes, normal music is played and then some numbers and letters are said vocally. This may actually be a normal radio station identifying itself. Other examples would be HAM radio operators, military or others who need to periodically identify themselves by law using their licensed call letters or call sign. This could be mistaken for Numbers Station activity by those who are new to listening. It takes time to determine if you are getting a genuine numbers station or just someone transmitting a music station or normal communications (if you hit it just right). Another example would be someone saying CQ DX a few times and giving their call sign (which is letters, or words that stand for letters and numbers) and even a country or location name. This is someone running skip or trying to get in contact with anyone listening in a country other than their own. This is not what is considered someone running a Numbers Station.
Now you probably wonder what all those numbers mean if you finally did come
across a genuine Numbers Station. Nobody really knows and that may be the point.
It is meant to be indecipherable to the general public and only knowable to the
Isn't that illegal? Well, not always. If it was some goof in the USA doing it for fun and intrigue and not making known to the public how to decode the message, then yes, it probably would be illegal (if not a grey area) since the FCC does not permit encrypted or undecipherable transmissions over the public frequencies (such as Shortwave and CB for example). However, military and government establishments all over the world are exempt so that they can communicate with staff to give mission-critical information to those in the field working for them. With the day of Cell Phones, you'd think they'd just send an encrypted text or by voice. Cell phones don't work everywhere. Using a shortwave radio, and even bouncing the signal off the ionosphere (aka skip or skywave) can be done with battery operated radios in remote locations, especially if they happen to also connect with a repeater somewhere to get their signals to go farther.
For more on the story, see The Infamous UVB-76 Numbers Station
You may come across other signals that sound rather strange. You can use the
cursor to tune into them, try a different band or mode and try to get a clearer
sound. You could also try using some filters that some webSDR receivers have
available. If the signal still sounds strange, you could try some of the links
mentioned in Identifying Signal Types to see if one of
those matches, and use a decoder to see if you
can get information out of it.
Be warned that decoding some encrypted signals may be illegal in some areas.
And in some cases the data is encrypted so even if you did decode it, it wouldn't make any sense. Be careful and try to decode only what you are sure is public. However, just learning to identify the signal type should not be too big of a problem, and good practice.