Transformer Wiring – How to Terminate Unused Transformer Pins

Normally this would only need 4 wires: the common, and taps for the three voltages. Having 6 wires complicates things a bit.
One way to find out is to measure resistance between wires. If there is indeed an common and three taps you should measure \$\dfrac{220}{120}\$ the resistance between 220V and common, compared to the resistance between 120V and common. So look for ratios \$\dfrac{220}{120}\$, \$\dfrac{250}{120}\$ and \$\dfrac{250}{220}\$. If you measure a relatively small resistance between pins, you're probably measuring between 220V and 250V.

Alternatively you can apply a low AC voltage (from the secondary from another transformer) to the secondary and measure voltages between pins, and find out the ratios. Even if you know the secondary's voltage and apply that, don't expect to measure 120V, 220V and 250V, since an unloaded transformer outputs higher than nominal voltage.

If you already have a ground connection with one or more other wires, then use the otherwise unused wires to make a more solid ground connection.

If there is otherwise no ground connect, like with ethernet for example, then I agree with Keelan in that you shouldn't create one. That could cause trouble with ground loops. In that case, either leave the extra line floating or connect it to ground at one end only.