Well, I'm no expert on hops as this is my first harvest year. However I can tell you what I've seen with my plants.
My centennial was the first to produce cones. They started to get big so I opened one up. The lupulin was visible but light yellow, the cones were green, and the smell was slightly grassy/very lemony.
That was about 3 weeks ago. Today I climbed up and picked the most mature-looking cone from the very top of the plant, which were the first to set. It was just started to brown a little on the edges, and was pretty much the only browning one I could find.
The color was much lighter green than the younger cones. The lupulin was starting to look orange-ish instead of yellow. It smelled WAAAY better than the younger ones. I think it was mature enough to harvest.
However, since it was the only one apparently this ripe, I'm leaving the plant alone for a little longer to let the younger cones catch up a little. Right now, unless I see some serious browning at the top, I'm planning on harvesting Labor Day weekend (two more weeks) and making a wet-hop ale with them then.
I have to say though, having not known the difference between under-ripe and ripe before today, the difference was extremely obvious. If you're not sure, wait.