Turn on Nuvi 760
Press and hold near upper right corner (by the battery symbol) for several seconds and release
This will put you into the maintenance mode
Now, plug in your USB charger
In the maintenance mode, you can scroll through the screens. One of them has a battery charging status in percent. You can watch it climb a percent at a time.
Normally, I just exit the maintenance mode by choosing exit near the bottom right corner. then the nuvi will reboot and will be usable just like you are in the car cradle while charging and the normal battery symbol can tell you the state of charge.
The maintenance mode is neat, just be sure to scroll through the screens and always use "exit" to leave it. To be safe, never switch it off from this mode.