If you have a multi meter check for continuity between each wire and the metal part of the iron, the one that shows a direct short will be the earth (green/yellow). The other two will be live and neutral and it doesn't matter which way round you connect them (though red will technically be the live)
Years ago, before the current standards, for British domestic leads, the wires were red (live), black (neutral) and brown (earth). I think you have a foreign soldering iron so be doubly careful or, as suggested, just throw it away.
Before you take a gamble, please follow chuck's suggestion and check for earth. You can even check with a battery/bike bulb and a piece of wire - although you then need four hands.
If it's foreign it may even be 110V too... although I guess it would do no real harm if it was, it would just burn out the element within a few seconds of plugging it in!
End of the day, the earth is the important one, that will be connected directly to the metal parts of the iron, it's only a heating element so it makes bugger all difference which way round the other two (live and neutral) are connected.
If you have a multimeter, measure the resistance of the element, you can then calculate the wattage at 110v and 240v if the wattage calculated for 240 volts seems too high for it's size then it must be a 110v iron. 25 watts is normal for electrical/hobby irons.