QUESTION:
Could the rover be on Earth in a desert somewhere with out you knowing it, 
and if it is on Mars, how do you know it really is?

ANSWER from Steve Wall on July 15, 1997:
Most basically we know it's on Mars because of where we have to point our
antenna to hear its radio signals.  We also know because the changes in that
signal, called Doppler shifts, match up with the speed of Mars relative to
the speed of the Earth.

There are other ways that we know it's not on Earth, most practical is that
we know we put it on top of a large launcher and we know we saw it disappear
into the sky.  It couldn't have landed on Earth because we tracked it the
whole way to Mars.