Lots of people on this site add ice to their runs during the summer. Some put frozen plastic bottles in their run, but ours never went near those.
When I see my chickens panting or holding their wings away from their body I know they are too hot. We put out spiral plastic sprinklers that spray a mist into part of the run. When we first did it they didn't like it, but it didn't take them long to decide they enjoy it and freely walk through it now. We also feed them watermelon as a treat to help hydrate them. It might be good to give them electrolytes in their water once in a while too.
My chickens experienced heat for the first time last week and they were not excited! If they're panting (walking around with their mouths open) and holding their wings away from their bodies (and overall looking droopy), they're too hot! I put ice cubes in their water to cool them down and sprayed them with the mister on the hose. Like limited25 said, a lot of people put little misters in the run during the summer. It also helps to add some extra shade to their run.
That having been said, being hot isn't necessarily a crisis! Cats and dogs pant when they get hot and I put ice cubes in their water during the summer, too. There's a difference between the lethargy chickens will get when they're hot and the lethargy chickens chickens will get when they're dehydrated. As long as they have shade and an adequate water supply, they should be fine. If you're really seeing sweltering heat, it's fine to add some ice to their water!