My Navman F20 won't switch on?

I have been given an F20 Navman that will no longer switch on. How do I check if this is because the battery is completely dead or because the 12volt connection lead is not working & has not charged the battery?.
When I plug into the 12v supply the green light comes on the charger and the fuse is ok, but how can I check the voltage is available at the other end of the lead at the D connector ?
If the battery is dead and the 12v lead is plugged in, should it power up & switch on anyway ?
Any help would be appreciated