What are the terminals of a 6 wired step down transformer?
It sounds like this transformer has a total of 4 windings.  How do you know one of them is intended to be the input and the others output?

Physical size will give you some idea of what power this transformer is intended to handle.  Since you gave no indication of that, there is little more I can say about it.

The first thing to do is to probe the wires with a ohmmeter.  At the very least, verify that you really do have 4 independent windings.  That means you should have some finite resistance between each wire pair, but infinite resistance between pairs.  If this is not the case, then you have multiple taps from fewer windings.

To find the winding ratio, put a small sine wave into one of the windings.  Use the "input" winding if you're pretty sure that's how the transformer is intended to be used.  A few 100 Hz to 1 kHz at a few volts should be fine.  Measure the voltage the winding is actually being driven with after it loads the signal generator.  Now measure the open circuit voltage of all the other windings.  The ratio of these to the voltage on the input winding is the turns ratio.  This open circuit voltage relationship will scale up to larger voltages.