Some of the modern ones are phased backwards at the factory, so you should always check them, but... in my experience, if the phasing isn't correct, the new transformers just go right into overload (breaker trips) as soon as you crack a throttle open... so not much need to check if it works one way and not the other.
I also had a pre war Marx transformer that in phase lights a bulb pretty well with almost 7v between hots . Thinking that was out of phase I switched the plugs and got about 36v.... and quickly switched back. Had I used a 6-12v bulb, it likely would have cooked it 
Also "the spark method" isn't very reliable for folks without the experience to gauge a sparks power. And it also isn't reliable because you don't always get a spark, and/or may get spark on both ways (thats where experience comes in; which spark was "stronger"?)... so, it works, but isn't totally fool proof....a volt meter or 14v-24v bulbs along with some good judgement are the way to go imo.
