Certainly using the micro switch/leaver switch is the simplest way. In case if you don't want to use mechanical approach, you can do this with IR proximity sensor as well.
I'll check that out, thanks.
Would be the cheapest route.
That is pretty cool, wasn't aware that was possible. Will take a look at that too.
If the lid is a swing style ( doesn't detach completely ) and object doesn't move too much, a mercury switch would be suitable.
Yeah it's a hinged lid, never heard of a mercury switch, I'll look it up.
Thanks for the suggestions everyone.