Ha! I asked exactly the same thing in ground school and ended up putting quite a lot of background work into finding formulae that would help me figure this. I finally got a working formula that I have built into a spreadsheet. Entering Field elevation, amnd the ATIS information including Temp and Dewpoint does the math. The sheet also does my performance, weight and balance, crosswind and reject take off speed/distance. Be happy to share but you need Excel 2007 as a minimum to handle some of the formulae which are not in 2003.
Or you could use this from this week's AOPA newsletter on the subject:
“Humidity should not be ignored but almost always is because aircraft performance charts do not include it as a factor, but should,” wrote AOPA Pilot columnist Barry Schiff in his July 2007 Proficient Pilot column. “Although even the most humid air is not that much lighter than dry air, it causes reciprocating engines to lose considerable power.”
He offered an alternative way to estimate humidity’s effect: “Because the precise effect of humidity involves complex calculations and is difficult to determine, pilots can compensate by raising the calculated DA by 1,000 feet on hot, humid days and decreasing calculated performance by a conservative fudge factor of 10 percent.”