Let's talk about Hunts - a detailed rundown of their assumed mechanics

[SamSmoot]

I just run around till I see one :-/

How do you know if you found one? I joined a group to kill one in Azim, and there was nothing unique about it other than a name. No A, B, S label, nothing. Could have just been some quest mob from it's general appearance.

[BillyKaplan]

How do you know if you found one?

One good indication is that you can see it from literally across the map. S ranks render regardless of distance so if you see a mob way way in the distance, that's one indication.
Another indication is that it spawns over you. Granted B or A ranks do that too when their timer is up, but that leads me to the next one...
They're huge. They will no doubt be the single largest mob in that zone. If you look at something and go "holy sh-- that thing's humongous", it's likely an S rank.

And then there's always the zerg rush to kill the thing, that's usually a good tell.