How do I determine what kind of threading I have?

Threading gauges! Joke apart, check this and Sheldon Brown's entry for threadings. For English threads you can count (with some patience) the number of peaks per inch as is explained in the first link.

Being probably a custom ordered bike, the type of thread used might have been the choice of the original owner of the bike: Any idea about how old is the bike? Spanish bikes, as far as I know, used either French or Italian threadings in the 70s and 80s. The French threading became relatively obsolete (apart from France) by the end of the 70s, as the ISO standard became the reference for the components industry. My bet is that, given Zeus reputation for Campagnolo knock offs, chances are that your fork has Italian threading, assuming it's the original one.

Hope it helps

Cheers ED 13! Very helpful.