It is a typical way of testing.  I'd add that the motor can torque rather forcefully and the starter needs to be properly held. When you say, "hooked up a cable" by what means were the connections made?  Was the cable bolted to the battery and starter both at the + terminal and when grounded to the starter case?  I've tested starters using jumper cables and have found that unless the cable clamps and place I'm connecting to are VERY clean I often don't get a connection.

But, frankly from your description of "getting sparks" it leaves me incline to say the current is getting through. I have found there is sometimes a delay (under a second) from the point of current application to the point of the starter solenoid engaging and powering the motor.  So, if you are merely tapping the connection for a split second you may not be getting engagement.  Otherwise it would seem the starter is shorted so don't hold the connections for longer than a (real) second as the overload can get dangerous.  Proceed with caution at your own risk.