how to determine the current capability of a transformer
The main determiner of transformer power rating is the material and cross-sectional area of the core. This sets the power level at which the core will saturate, and manufacturers don't want to use any more steel than they absolutely have to.

As a rough estimate, since these transformers generally use the same kind of steel laminations and the same relative geometry, it is sufficient to make a judgement based on the volume/mass of the core. Since the core is the majority of the total mass of the transformer (the windings are a tiny fraction), simply weighing it is enough, which is why Marko made his comment.