First, you have to sum the VA (volts x amps for a purely resistive load) of all the secondary windings. For example, using a transformer with a center-tapped secondary, measure the AC **current** and the AC **voltage** of one secondary side, using the center-tap for the ground reference. Multiply these together. Next, do the same with the other secondary side. Add the results and that should give you the total VA. Compare this with the transformer's power rating. (You should be feeding a sine wave through each channel with terminated outputs while taking measurements.) You can measure current by putting a meter (set to "amps") in-line with the secondary being measured. You'll get most accurate results using a "True RMS" meter.