graham, have a look at the pcb itself, they should be labelled in some way.
*edit* sorry that was pretty blunt.. apologies.
Look at the insertion point of the wires onto the solder pads (for strip) or the pcb (for strings)
There should be some sort of marking there to determine what is what.
1 thing I will say is dont believe the colours of the wires.
Normally, the outside wires will be your +ve and -ve, i do say usually however. so going by that logic, the middle one (green) should be data but, check.
Another way to test is use a multimeter, attach at one end on a colour wire, then attach at the other end on the same coloured wire. If you dont get a through result, that is a line going through an IC. more then likly a data line.
In theory you shouldn't work from the wire colours as they can be all over the place. It's hard to get quality control when you pay someone a few dollars a day. Providing the pixels have clear bodies the in and out directions and the 3 connections should be marked on the embedded pcb. Failing that a continuity test from 1 end of the string to the other will tell you which 2 are the power (they are connected straight through from 1 end to the other) and the other 1 is the data.