You can safely test it when switched off/unplugged with a multimeter set to ohms/continuity range - probe between all unique combinations of pins and note down which (if any) are connected to each other (reading around 0Ω if set to Ohms or it beeps if set to continuity)
After this test on DC volts range - start at the highest setting and work down (or set it to autorange if it has one)
Test all unique combinations of pins, you should be able to deduce which pin(s) is(are) ground and what voltages are present.
Be sure to note the polarity of voltages when testing.
For example, if you find (only) pins 2 and 4 are connected together, and testing from either of these to pin 1 produces +5V, and testing from either of them to pin 3 produces +12V, then you have a dual output (+5V and +12V) supply with a common ground.
If you need help deciphering the results, update your question with findings from above tests.