theres 2 simple ways,one is unplug the speaker and put both test light ends on the leads,put the fader to that speaker and turn it up,it should lith the tester,ot you could use a volt meter and read the voltage.
To test the speaker get a ohm meter and check the resistance across the contacts.
You can check it with ohm meter, or continutiy, but if you have a damaged voicecoil it may read ok with the meter but when you move it a little you may find a break, like others said check all cable and connections, you could always swap in a extra speaker just to see if it works.