Look at the volume. When the volume in the front month is less than the volume in the next month, that is when the rollover is complete and it's time to change contracts. For example look at the volume for Crude Oil on the CME (CL). On Thursday Feb 14th, the March contract still had more volume but on Friday Feb 15th, the April contract has more volume. Time to trade the April contract. The April contract had more volume at the beginning of the session on Friday so from Friday you should have been trading the April contract. You only need to compare the volume between front month and next month near the rollover dates:
https://www.cmegroup.com/trading/equity-index/rolldates.html
https://www.investopedia.com/university/how-to-trade-e-mini-futures-contracts/beginners-guide-emini-futures-rollover-dates-and-expiration.asp