As explained in this article, a decent radio range is important for timely warnings and good situational awareness. Installations can deteriorate over time, e.g. the cable breaks or connectors get loose. It is thus good practice to regularly re-verify the range of every FLARM installation.
A simple and effective test is our online Range Analyzer. The analyzer requires one or multiple flight logs in the IGC file format, as recorded by the FLARM device. It uses data from received traffic for the analysis, hence it requires an abundant number of traffic encounters during the flight to generate meaningful results. Hence a log from a nice summer day with a lot of activity is better than a log from a night VFR flight
Another means to check range is the KTRAX Live Range Analyzer. The analyzer works on data received by the OGN network of receiver stations. It will thus not work if flying through areas with no OGN coverage, or if privacy options (notrack, random ID, entry in DDB) are active.
Also read our FTD-041 document on the topic.