Follow SCG Tournaments and MagicFests/Mythics results to see if any breakout decks dominate or if a subset of particular cards appear in multiple decks. A good example is the ProTour Rivals of Ixalan (when Mythics were still called ProTours) where [[Collective Brutality]] was in a number of Top 8 decks. You can even see the spike right after on MTGGoldfish.

Youtubers like Command Zone + MTGGoldfish (SaffronOlive) can hype up cards if they do well on their shows. I feel like the 'SaffronOlive effect' hasn't been common as of late, but the Command Zone has a knack for spiking cards based on how well they do. For example, one of their latest Game Knight's episodes featured an colorless Eldrazi deck with [[Kozilek, the Great Distortion]] as the commander, and a few days later it nearly doubles in price.

Honestly,if you just play the game and follow the meta (and the hype) you'll start to see trends and you'll start to get ahead of the curve.