OK then another idea, put a 10k resistor in series with the led, connect it to 5v or better 10 or 12v and measure the voltage across the led's connection terminals, f its around 1.8v for red or 2v for green then chances are there is no built in resistor, if the voltage is much higher and the led doesnt light or isvery dim then theres a built in resistor.
Not definate but a bit closer.
5v across a red led might well blow it in a mS.
Edit looking at the datasheet I'd say the resistor was built in as they quote 12v and 220v dc.
Might be worth a phonecall or email to bulgin, they are a well known company so probably will be helpfull.