Sometimes you can compare the size (weight) of one transformer against another of the same manufacture and this will let you know the VA rating. In other words the "watts" of the transformer equated to AC values. If you have a similar transformer and it is 15VA, then the transformer is a 15VA type.
If the output voltage is 15 volts (AC), the current will be 1 amp.
Similarly, you can measure the gauge of the secondary winding and if it is similar to another type that will deliver 500mA, the same applies. This is only a guide so that you don't think 500mA from one transformer will deliver 2 amp from another, with the same gauge wire.
If you only have a DC meter, put a bridge on the output and a 1,000u electrolytic and you will get a DC voltage appearing across the electro.
A 15v AC transformer will produce about 22v DC from the bridge.
Because the voltage is increased during rectification, the current capability must be decreased accordingly so that the final result is 15VA. The current will be about 700mA.