IB does not have a Prem symbol i programmed one for my charting program, but IB should have one already imo
IB has something called the "Index Arbitrage" meter which measures this premium, which you can get by entering spx and selecting "index" (on my TWS, it popped up another window showing the "arbitrage meter") Unfortunately, you can't create generic combos using indicies, but you could play around with the premium between SPY and the ES futures, as an example. Create a generic combo with Buy 1 ES, sell 500 SPY, and you'll get the premium of the ES contract over the SPY ETF. Whether than will help you is another story.
it is set pre open every day. Go to indexarb.com for the offest (premium). The delta of the offset is a leading indicator of price given that there are four combinations of possible events. A continuous data feed is required and it must be coded to provide an intelligible display. There are about four approaches that are useful for monitoring for making money. The incidence of the signal is 30 to 45 seconds, easily, before the trade is put on or other leading signals indicate a trade is coming up. The persistence, after incidence, of the signal is in the 3 minutes range before there is mostly just an appearance of decay. The best pair of values that are usable relate to smart money. See some discussion on this elsewhere. The signal is mostly only useful to expert traders as a consequence of where it fits into their data taking.