Well since we don't know, we can always make up something interesting.
Here's my thought: Perhaps they captured some orcs and did some biological experiments on them. Then they saw some ligtning bugs and thought of the biolumensence thing they do. They made a concotion of chemicals that would react to Orc flesh or blood, sweat. etc. when its near so that would cause this new chemical to glow. They coated their swoards in the stuff and Voila! they have Orc detectors.
don't forget, this is only my wacky theory; Tolkien never stated this....
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'They say that the One will himself enter into Arda, and heal Men and all the Marring from the beginning to the end."
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