The easiest simple check is to measure the voltage at the battery under various conditions. With no charging going on but a charged battery, it should be somewhere in the vicinity of 12.5 volts or a bit higher. When the converter is on (and the battery is basically charged), it should jump up to perhaps 13.3 to 14.5 volts, depending on the converter model and (if a multi-stage converter) what mode it's in. With the engine running, it should likewise be around 13+ volts with the alternator providing power.
For a more thorough check, you'd have to discharge it some and monitor the voltage and current from the charging sources...but a quick voltage check will at least tell you if the connections are in place and the system appears operational.
If you have one of the little four LED battery level indicators, the top LED might be off when the battery is not being charged but should be on if it is receiving a charge. These little indicators are not very precise or accurate; on mine, the top light is often on just from the battery power with a fully charged battery.
