To be safe if you decide to connect the transformer to the mains, use an incandescent bulb (say 100W) in series with the transformer. The resistance of the bulb is low enough to pass the normal transformer magnetizing current and will allow you to measure the output voltages, but will prevent anything bad from happening if you have the wrong connection.
The light should glow dimly, if at all, if you are connected to the correct transformer wires. If the bulb lights fairly brightly when connected, then you have the wrong connection.