How did Marshal Pentecost know he and Chuck Hansen were drift compatible?

It is established right away that drift-compatibility is something that is based on a kind of equality in spirit as it exists between what one could more poetically call "soul mates" (like Raleigh and Mako) or brothers (like the Beckets) or father and son (like the Hansens). So given that Marshal Pentecost and Herc Hansen were once drift-compatible and Herc and his son were drift-compatible it is not a large stretch that there is a high chance for a transitive drift-compatibility between Stacker and Chuck.

There is on the one hand of course a genetic compatibility between Herc and his son, which Marshal Pentecost could have meant with the sentence "You're your father's son". But even more likely he probably meant to say that Chuck is in fact pretty equal to his father in his spirit and mind, or maybe how his father once was in his youth (or when he was riding the Jaegers together with Stacker), or said more colloquially that "the apple doesn't fall far from the tree".