Smell them. If they have a hop smell they are probably ready. Mine haven't developed as great of a smell as they have in previous years, but I know they're fine.
Also, if you squeeze them, you'll notice they feel very papery and springy. If you break them open, you should see lots of yellow lupelin on the leaves. These are all signs they're ready.
If you're worried about knowing the right time... wait until one turns brown (don't use this one as brown means bad). Once one turns brown, the rest will be perfectly ready.