As a man who almost never pre-cums but, admires all that do...and keep men that do around for as long as I can! I have always noticed pre-cum to be very clear and extremely more slick when rubbed between your fingers than actual cum and ejaculation fluid. I have found that when a man begins to flow you can milk it up and out by rubbing and lightly pulling from the base of the shaft up to the head, causing any fluid in the uretha to drip out.
But, the biggest flow I ever saw was while I was doing partner yoga in a naked yoga class. My partner (a stranger prior to class) got very excited without any visual sign of excitement, a strong flow of pre-cum oozed out him and down onto my leg. As I looked he stood again, as I saw a heavy flow roll from his penis head to the floor. He shyly placed a small towel infront of him and broke off the pose. Was very exciting, even thought he was unnecissarily embarrised.
We never spoke of it.