Well,
In order to know what can be stored in a capacitor,
you only have to look at 2 things,
both are written on in: voltage rating and capacitance.
You should assume it is charged fully!
Q=C*V
A.I don't recommend discharging batteries or super-caps,
just be very careful with them(don't short them...ever!)
B.Don't try anything with very high voltages- above 500V !
(that is the realm of pro's).
C."AC caps" in Air-con/Motors equipment can hold charge and voltages
up to about 500V(depends on mains voltage).
How to discharge
0.Use insulating gloves and wear "rubber shoes" for protection
when dealing with voltages above 48V(that is potentially deadly).
and don't stick your face near the cap.
1.Disconnect the equipment from the power source.
2.Connect a voltmeter to the cap's leads with probes
(leave it connected till the discharge is finished!)
3.Connect a resistor,a few hundred ohms several watts
(preferably use a discharge prob) across the cap.
Monitor the voltmeter till you get about zero volts.
Here is a how to discharge video
