Hi Mark,
not quite as you say.
Connecting two traces on different layers the via will never have a name. Using the "Show" function you will see if the net name has changed (e.g. from VCC to N$1) which will be the case if you place a via on two different traces, not having hit the exact coordinate.
Renaming the false net into the original one you'll see an air wire, very often just a dot in the center of the via, indicating that the traces are not connected properly. Use manual routing to connect traces properly. Eagle also generates a sound signal when both traces are connected.
Regards
Boncuk