There are screeners and filters out there to compare period x to earlier period y. This is volume and I would think , (incorrectly?) that the +correlation of volume and AUM would be strong. Reading 5-10 popular etf would possibly keep you in time with popularity changes. Writing a key word scraper , some kind of heat map, would possibly alert you to changes as well. Finding large relative volume changes beneath the radar , in obscurity of the etf world , alined with failure of a word scraper is what I might look for given the inclination and talent.
Best I could find that's free: Daily top ten creations/redemptions: https://www.etf.com/sections/daily-etf-flows/etf-fund-flows-2019-03-14 Weekly top ten creations/redemptions: https://www.etf.com/sections/weekly-etf-flows/weekly-etf-flows-2019-03-14-2019-03-08 Like someone already mentioned; the real deal is not free: https://etfdb.com/screener/#tab=fund-flows
If it helps, a good screener will show you all ETFs by volume, as in the example below for today 3-21-19 showing top 20.