[RichardJ]

>>how do i check them while the connectors are still plugged in?<<

It's called back-probing. Simply slide the voltmeter probe along side the wire into the back of the connector. Easy with these type of plugs. Much harder with newer type plugs such as the '80s weather-tight plugs.
When the plugs are too tight, use a straightened paper clip to slide in alone side the wire. Still not enough room, use the straight pins you saved from a new dress shirt. As a last resort the pins can be used to pierce the insulation at a low angle. The insulation will usually pretty much self heal.
I have alligator clips that slip over the meter probes, but I bought most of them at the now defunct Radio Shack.