Okay, so as far as knowing when your chickens are close to laying. Your chickens are about 13 weeks old now, so you've got a little ways to go still. You should be looking for eggs around the end of Sept.
Remember that a chicken's laying cycle depends on the hours of sunlight in the day, and that they need about 14-16 hours of light to produce eggs. A lot of people rig up a light (a 60 watt is more than enough) that turns on in the morning, since it's easier for the chickens to wake up suddenly than to go to bed. I learned that the hard way when I cut the light out for them after it got dark out and they all bumped and flopped around trying to get up on their roosts in the pitch dark! LOL!!!!
When your girls are getting ready to lay they get pretty loud. They'll squawk and screech like old hens (sorry, couldn't help it). Also, if you feel their back ends right below their vents you'll notice that their pelvic bones are very close together. When a hen is ready to start laying those bones will spread apart. I would start checking around 18 weeks, cause depending on the hen and the breed they may begin laying that soon. Hope this helps!!!
BTW: I forgot to mention that it is ILLEGAL to post about specific animals on this site without additing PICTURES to your posts. No pictures is the absolute worst travesty that can be committed on this site....(not really, there are a few others....death threats, etc...LOL!!!) Seriously, though, pictures are a must!!!