Another (similar way) is to measure the size of the transformer (as above), then look at transformers in a catalogue for one as close in size as possible - it's likely to have a similar wattage rating. You can do the same thing by weight.
Bear in mind, these crude methods give an indication of the total wattage of the transformer, if it has more than one secondary winding this wattage will be shared between them - how it is shared is dependent on the thickness of the wire, and how many turns there are.