Hmm; good question.
Well you see there's several ways: For one, If I pick up a series, and start reading it early on and it just doesn't feel right, that's a drop. That said, sometimes I push through, try to find something to latch onto; Seven Deadly Sins and Fairy Tail are good examples. I found hooks sure, but at the same time I got to chapter 40 in 3 months, while I cleared all 220 chapters of (Yes I'm mentioning it again) Vinland Saga in a week.
I tried the same thing with the Ravages of Time and Kengan Ashura; unconsciously somehow, chapter 40 is the make-or-break line for me, and If it takes me forever to get there, that means it's not worth struggling through more, it's just not my thing.
This just reminds now that I should try Kingdom again; I got 300 chapters in and dropped it because of how blue things got anytime the mother appeared... which is a bit hypocritical when I think Souten No Ken, Shadow Warrior, and Keiji are up there and were far more... Blue in some sections.
PLUS, when I read Vinland Saga, while the stories and art are different, I couldn't help but think of Kingdom; the environmental detail I think. Of course, Kingdom leans far deeper into the supernatural, while the most supernatural thing in the former is the mushrooms Bjorn takes and the visions the shaman had toward the end of the series.
I heard they're starting to release volumes in english too! Exciting! Kinda sad they aren't like big hardcovers.