How to check if a DC power supply is a floating source?
There are a couple of ways that you can verify if the power supply is floating.

1)  The easiest method involves using a piece of commercial test equipment called a "Hi Pot Tester".  This is a device that provides a current-limited high-voltage (either AC or DC) across the device under test.

You simply short both of the output DC terminals together and connect to the Hi Pot Tester Ground lead, then short both input AC terminals together and connect to the Tester HOT lead.  Apply voltage and ensure that the Tester does not show excessive current.

You should follow the test procedure that your country's Electrical Authority provides for the proper usage of the Hi Pot Tester.  In Canada, our test voltage is (Two times AC Line Voltage) + 1000V for one minute or about 1250 Vac for one minute.

2) Simple method but not as thorough as (1) above: Connect a pair of identical resistors in series and connect the free leads of the resistors to the power supply output terminals.  Connect the middle connection of the series resistors to earth ground.

Measure the voltage across each of the series resistors.  The voltage should be identical across each resistor.

Pick the resistor value such that you get about 100mW dissipation in each of the resistors.