Never been used eh?
Gas: Look for pipe dope in the threads of the inlet to the gas valve (this could be evidence of prior hook up). Actually, if it's brand new, there is a plastic pug stuck in the valve at EOL/ assembly.
Electrical: Look at the wires that you hook the unit up to high voltage. If brand new, the wires will stick out of the j-box about 10 inches (You will need to bring a 5/16 and 1/4 inch nut drivers to get into the housing, and then junction box on the heater). You will be able to tell if it has been hooked up to power (wire strands will be wiggly and not straight).
Water: The water connections are union-ed 2" and should be ready to accept pipe, not have a piece of pipe already glued in then the pipe cut off.
And general cleanliness.
For 900.00 i wouldn't buy it if it had water running through it. Even if it wasn't ever fired. There is no way of knowing what the water chemistry was. If it had water that was for instance say low Ph, copper would have been eroded away, reducing the life of the exchanger (worst case of course).
Good luck