Since dry potting mix is much lighter than wet, you can tell if your basket needs more water based on its weight. Just lift the basket an inch or two to determine how heavy it is.
When watering, you should fully saturate the soil until you see water spilling from the drainage holes. This will provide the needed hydration and prevent fertilizer salt buildup.
For a low-stress solution to your hydration problems, set up a drip irrigation system to ensure your hanging baskets receive consistent moisture throughout those hot summer days.