You need to get yourself an inexpensive digital volt/ohmeter. Please click the link at the end of this message for a typical one. They can be had for around ten bucks, and will serve you well in all your electrical tasks. Once you have one, put the rotary switch in the correct position for the voltage range you are about to test. You take the red lead and plug it into the + jack. Take the black lead and plug it into the - jack. Now, when you put the red lead on a positive source, and the black lead on a negative source, the meter reading will indicate the voltage, with NO negative sign in the display. If you have the leads reversed, you will get the voltage reading with a NEGATIVE sign before the voltage display.
If you use an analog volt meter, putting the leads in the correct places will result in the meter needle moving to the right. Putting the leads in the reverse places will result in the meter needle pinning against the left stop, ultimately damaging the meter movement.
If you know what a diode is, and how it works, a simple polarity checker can be made with one diode and one small lamp in series. Cost: about a buck total. But I highly recommend getting a meter.
I'm surprised that the power supplies you have are not marked, somewhere, somehow, with their output polarities. There is no such thing as a "D-C transformer." Transformers are used for A.C. and would not have polarity markings on their outputs. Maybe the items you have are transformers, and not D-C supplies??? Can you tell us the manufacturers and model numbers? A photo would help, too.
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