The following is where in-development documentation will appear for TidalCycles 1.5.0 and later. For older versions, please see Custom OSC (pre 1.5.0).
One way to debug OSC is to use a packet sniffer like wireshark. You can put "osc" in the filter field (no double quotes) to filter out everything except OSC packets. If you click on an OSC network packet and expand fields you can find a nicely formatted representation of your OSC message.