Use a voltmeter to check the voltage on the starter's solenoid, with the ignition in the "off" position (should read 0v) and then on the "on" position (should read close to 12v). If you're getting some voltage but not close to 12v, it's probably your battery. If you get nothing, then check the fuse and relay. As a last option, you can remove the starter and connect it directly to a 12v source and see if it works. Be careful when handling the starter outside the vehicle. You may be able to do this with it still mounted on the motor but it's a bit more difficult and somewhat dangerous.