Alternator problem 01 blazer
 

Today the alternator/battery light came on in my 01 blazer and the voltmeter was sat at 0, now the voltmeter has occasionally sat at 0 before without the warning light being on which I put down to maybe a sticky/iffy gauge. Is it likely that my alternator has breathed its last or is there something else taht could be causing this like a charging relay somewhere? The reason I ask this is that just before I got home after the light came on the speedo suddenly went nuts bouncing up and down and several other warning lights flashed on and then it all went back to normal after a second or two.

Any help appreciated, it appears to be a one wire alternator so how do I test this, I have a meter.