Very simple. Pretend you own the stock. A lot of it. Watch the chart. Is the downturn sharp with a lot of volume? Its called capitulation. Are you feeling serious pain? Are your hands getting clammy? Is your heart racing? Do you feel like throwing up? Need a drink? Well, might be a good time to think about buying.
NYSE TICK gets to -1000, your most likely gonna get a bounce. It's the only known leading indicator I'm aware of. I use it religiously for ES trading and it's consistently helped me make good money. Like anything, it's not 100%, but it's damn close.