Fair value is the relationship of a market index to its futures contract.
If the futures are higher than the fair value, then traders are betting the market will open up. However, if the futures are lower than the fair value, traders are looking for the index to open lower.
This is not a precise measure and it will change during the night. The closer it is to market opening, the more reliable this gauge becomes.
You will hear the electronic media reporting this relationship before the market opens or you can go to any number of sources and get the information yourself.
The accompanying chart comes from CNNMoney.com.
It shows that at this point in time, traders are expecting the S&P 500 to open slightly higher and the Nasdaq to open lower.