Firstly, forget about nailing the bottom, or top. It is impossible and not worth the time, effort, and frustration.
Next, you should not depend on nailing the bottom or top in order to generate profits. Extract a part of the total trend up or done and you would be profitable.
So, how could we identify the top or bottom, bearing in mind we cannot pinpoint the exact top or bottom?
Price: An 18% down week followed by a 5% up week is rare; the bottom may have been made! Was the decline to 860 last week a retest of the low? Most likely. Looking back at previous bottoms (in a post in ET somewhere), it would appear that the markets may have bottomed. However, it will not be a meltup from here but a back and fill process. Patience is required, Dramamine also, and be alert to the fact that this may not have THE bottom.
Sentiment: to me, this is crucial. VIX has spiked up to record levels and then retreated, by not by much. VIX closing below 50 would indicate a reversal in progress. Some prefer looking at a 5-day RSI of VIX, for oversold VIX below RSI=20, and then a bounce. Big negative ticks, big range bars, extreme A/D and Put Call ratio, high volume declines are also other useful indicators.
Most important, traders should become very frustrated, almost suicidal (but we don't want them to jump off the Brooklyn or Verazzano), majority of participants become totally disgusted with the markets, some stopped trading, front page newspapers and magazine covers abound with stories and pictures of doom and gloom in the markets, everyone thoughout the world blaming America, and politicians blaming everyone but themselves and holding inquiries etc.
While all this is taking place you look for opportunities, raise cash, keep cool, then buy - maybe 25%. Wait a while for back and fill, retest, and then buy again - another 25%. If market makes new low, don't panic, wait for extreme oversold and buy - another 25%. Hold. Let the markets settle and reverse and rally 15 to 25 %, then take profits.