Distance from/to an airport (or other places with an appropriate antenna) is measured rather unsurprisingly with the Distance Measure Equipment (DME). This is the most easier navigation tool to operate. This tutorial is for FSX, but the DME works exactly the same in real life or FlightGear:
https://www.youtube.com/watch?v=mPy15TC-h9sThere are many ways to learn the DME frequency for an airport, if the airport has it (most medium and large size airports have DME) The most obvious way: it is in their real-life charts. Unfortunately, these charts are not always available for non-pilots. You can check the map inside FlightGear, but I find this map a bit difficult to operate.
You can also check SkyVector. For example:
https://skyvector.com/?ll=49.9983826203 ... 301&zoom=2 That's Frankfurt, Germany. Do you see the large blue "compass roses"? They are VORs, and some of them are also DME. CHARLIE is 115.35, RIED is 112.2. Also, if you click on the airport, you'll find that the DME for Frankfurt is called FRED and its frequency is 115.90
For the symbols, check for example:
https://www.faa.gov/air_traffic/flight_ ... oducts.pdf (page 23)
FlightGear database is not as updated as SkyVector is, but most frequencies will also work in FlightGear.