Hi,

I have Garmin 640

Turn on your GPS.
On te top center there is an icon with a boat or a car, depending in which mode you are.
If you are in Boat mode, touch the boat icon, this will change the mode to land mode.
Once in land mode, touch and hold down the battery icon for 10 seconds.
This will enter the gps on test mode.
on botton right corner push next to times (third screeen)
You will see tha size of your sd card and the SN (serial number)