You need to turn off all electric switches, then disconnect the battery positive. Put a multimeter, between the battery + lead and the + terminal.
There will be a reading of current flow, if something is drawing current. Flat batteries are often caused by faulty boot/bonnet lamp switches. Some stereo systems do not power down completely, so draw a little current. Land Rover products, like Freelander and Discovery, do not shut down secondary electrical systems, until the vehicle is locked. Leaving one of these unlocked, in your garage, can cause a battery to be unable to start the car after two or three days.
A car battery in any car would drain due to a short in the electrical system.
If the ashtray light is not working it will not drain the battery.
You can get a car alarm that uses it's own battery so wont drain the car.
A starter does drain a battery. The battery holds a charge so that you can use your starter to start the car. After the car is started the alternator recharges your battery and runs your car and all it's accessories. If your car won't start or is having a hard time starting you can drain your battery by repeated unsuccessful attempts to start it.
If the alternator is installed incorrectly, yes, it can drain the battery.
Yes, if the relay was stuck it would drain the battery.
No.
leaving on the radio
Yes
only if the car your using to charge the other on is not started
Yes GPS is plugged in and comes on all by itself, it can drain the battery.
If the outlet is powered with the key off, then yes it will slowly drain the battery.