The starter earths through its mount through the engine SO check the battery negative lead to the starter casing with a multimeter to make sure that you have a good earth
OR
Use a jump lead from your negative terminal on your battery to a good earth point on the engine - if the starter operates your engine earth strap needs sorting out
simple way of testing this out just short the 2 terminals out on top of the solenoid if the starter works the solenoid is knackered if not itll be your engine earth strap