[i_am_proteus]

Can't echo this enough. If you're working on anything that has been connected to residential mains... use a multimeter. They're cheap! Check the power supply for residual voltage from PSU caps. Discharge them safely before working on the equipment!

[maayank]

How do you discharge them safely?

[i_am_proteus]

The correct answer is "bleeder resistor."

The other answer is "pliers."

(Make a connection across the terminals that have a voltage difference)

[avidiax]

If the maximum voltage is within range, a neon line tester with the button grounded should work well. When the light goes out, the remaining voltage is below the sustain voltage of the neon lamp.

That kind of tester is usually built into a screwdriver and often comes free with some electrical items. They are maybe $2 otherwise.