Here are two ways to discern if a Galvanic Isolator (GI) is fail-safe. First off, age, fail-safe GIs only came into existence in late 2008/early 2009. Second, from experience, all fail-safe GIs are labelled as such. Unfortunately there is no way I know to technically discern between a fail-safe GI and the standard GIs of past.
In terms of testing a GI, you should use a multi-meter. Remember safety first with AC! If the GI has failed closed you'd be able to measure a close circuit with the ohms mode. When working properly you should be able to get a reading while in diode testing mode. If failed open, you'd get no reading even in diode testing mode.