I'm in NW Wisconsin and harvested my Cascades late last week. I am guessing you should be close to ready if you are in Illinois.
There are lots of ways to know they are ready. There is the:
1. Squeeze Test - squeeze the cones and they should feel papery. Also you may hear some crunching.
2. Lupulin Color Test - take a cone and tear it in half lengthwise. The lupulin should be a deep yellow color
3. Aroma Test - take a whiff, it should smell like hops.
4. Browning Test - often when they are ready to go the edges of the cones will start to brown a bit.
5. The Scientific Way - Harvest some hops and weigh the sample. This is your wet weight. Take that sample and then dry it until completely dry and it isn't losing any more weight (you can use your oven on low or even a microwave). Now weigh your dry sample. Take your dry weight and divide by the wet weight which will give your moisture. Now subtract that number from 100 and you’ll have your dry matter weight. Once your hops get to 70-80% moisture content then it is harvest time. This only makes sense to do if you have a ton of hops. Otherwise any of the other methods work just fine for homebrewing.