Owning the index future is economically the same as owning the stocks with borrowed money. The future should therefore trade above the cash index by the financing cost and below it by the dividends foregone. When rates exceed the dividend yield, fair value sits above the index; when dividends exceed rates, it sits below.
Pre-market television quotes "futures above fair value" as a prediction of the open. That is a statement about the basis, not about direction, and the difference is usually small compared with the overnight move itself.
Fair value also decays to zero at expiry, which is simply convergence applied to a cash-settled index contract.
Example: cash index 5,000, 30 days left, rate 5.0%, dividend yield 1.4%. Fair value = 5,000 + 5,000 x (0.050 - 0.014) x 30/365 = 5,014.8. A future at 5,020 is 5.2 points "rich", which on es is $260 of theoretical arbitrage per contract.
Related: index-arbitrage, es, cost-of-carry, convergence, equity-index-futures