Starting with a charged battery (charged externally, e.g.) compare the voltage at the battery with shore power plugged in vs. unplugged. If you're seeing a significant difference on the order of 12.5v unplugged vs. 13.5v plugged in, then the converter is working. If there's no difference, the converter is dead or not hooked up properly. If you see some difference but less than 1v, the converter isn't quite dead but it's ill.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill