take a long cable wire, peel the tip as seen cooper, 1 to case (part case conductive / or back case) and 1 to the ground or wall , and now you can try catch it with your finger
The computer case should be grounded otherwise you will get a shock as the case will be floating at half mains voltage with a charge depending on the capacitance of the mains switching transformer.
Every computer power supply cord I have ever seen is 3 pronged and grounded. I would get a low cost outlet tester as well to be sure the power source in properly grounded.