I nailed the timing on when to draft Gurley (2017) and Cook (2019) when they were on my do-not-draft lists for years prior, they were round 2-3 guys that I knew would be round 1 and I overpaid. If you'd like I can do a more in depth post on how to spot such trends. I'm much better with receivers but I tend to hit it big with the RB1s as well.
The short version of it is keep an eye out on who the new coordinators and coaches are, look at the types of plays those coaches and coordinators previously ran with their old teams, look at how practices are going before pre-season to see if they are running the same schemes, look at every pre-season snap for the player you are eyeing to see if they are putting it into action. Also looking at o-line acquisitions and rankings is the other 50% of the picture when ranking running backs in your tier sheets.