Power supplies can be very dangerous. This is specifically true of high voltage circuits, but anything over 12 V must be treated as lethal potentially.
A power supply is not safe after it has been switched off. Filter capacitors can hold charge for the long time. In the high voltage supplies of good design, bleeder resistors of a high ohmic value are connected across each filter capacitor, so that capacitors will discharge in few minutes after supply is turned off. But do not bet your life on components which might not be there, and that can and do sometimes fail.
Figure-- A bleeder resistor, R, allows filter capacitors to discharge when a supply is shut off.
Most of the manufacturers supply safety instructions with equipment carrying hazardous voltages. But do not assume something is safe only because dangers are not mentioned in instructions.