for the capacitors - its pretty straight forward. See that strip of (-) signs down the sign. When you use an electrolytic capacitor that means that leg has to be the lower potential, so in this instance that refers to ground.
As for testing ground - easiest way is to use the blanking plate as it should be grounded. The problem is that sometimes (its rare but does happen) not all the grounds are "joined up" when the card isn't connected in the motherboard as the PSU will provide a common ground. On your multimeter there usually is a continuity tester which makes your multimeter buzz/beep when the probes short.Use this to check the grounds ;)
The inductor/choke will always be a good starting point for finding a voltage, but you have to be careful which leg you measure from - get more info here ;) From that you can usually see where the points all join up.