I presume you have the old jack which you've chopped off. The easiest way is to use a multimeter to test which colour wires go to which contacts on the jack plug.
Failing that, this Wikipedia article lists the many ways a jack plug can be wired. Hopefully you can narrow it down very considerably.
You will almost certainly find that all the plain colour wires, or maybe all the stripy wires, go to the jack plug sleeve, forming a common electrical return path.
The wires are usually insulated with solder-through enamel - slowly feed the end of a wire into a blob of molten solder on a hot iron and the enamel should melt, allowing the solder to flow onto the exposed copper. Unless you do this you probably won't be able to make a good contact with the multimeter.
If you're still not sure which pair of wires is which, briefly touch each pair onto the terminals of a 1.5V battery. You should clearly hear a click or scratching noise in the left or right headphone as you touch the battery on the correspondence wires. The pair that makes no sound must then be the mic.