How do you know the primary or secondary side of a transformer?

The primary side is usually labelled 'H', the secondary is often labelled 'L'.

Primary wiring will often be smaller, since the current carrying capacity doesn't have to be as high on the primary side.

The best thing to do would be to check with the manufacturer. They should have documentation stating what is primary and secondary.

The above answer only applies to step-down Transformers. The simple answer is that the primary winding is the winding connected to the supply, whereas the secondarywinding is the winding connected to the load. These terms have nothing to do with voltage levels.