Usually you will hear news of a bailout and the president will come out and tell people everything is okay so buy buy buy!
That's what Bush said a few weeks after the 9/11. That's what Obama said on the first friday of March 2009. Before the president speaks, the FED chair will give hints as well. I'm too young to remember what happened during the savings and loan crisis of the early 90s but I'm told something similar happened.
Just buy and hold and hold and hold. Crashes tend to be short lived. Recoveries take years and years.
Of course if you are greedy/ambitious, you could try to make money on the way down as well by shorting the market. But that's for the degenerate gamblers. Don't do it as it's nigh impossible to predict when the markets will crash and you may go broke betting against the market.