Well, if everything is unhooked, there are only four things in that circuit. Two diodes, a big-ass cap, and a resistor.
Pull the bottom cover off of the unit. You can see four diodes on one side, and you are interested in the two that connect to the 100,000 uF cap (the cap is labeled on the top side), but you can just check all four. Set your meter to measure resistance, and then measure across each diode one way, then the other. You will get two different readings, and the two readings should be consistent across all four diodes.
The resistor is across the 100,000 uF cap. Check resistance, should be 50 ohm? (Not positive, should be printed on the resistor).
If these are OK, then probably is the big cap. Bob Roberts sells them--might was well change both while you're at it. $35/pair.