What will a sell signal be for the markets?

You can see that the $SPX (top chart) has already risen above and fallen below it's exponential trendline...The $NDX needs to do the same (that is, fall below the line), then you have a 'sell' signal overall...

To be really safe you would like to see some real volume associated with the selling, and a good entry point (for a short fund) might be when the $NDX goes as far below the line as it went above it. Currently, that would mean waiting until it fell below the bottom on Monday, below 1900 or so...