The Astros hid behind the guise that they were outstanding at picking up "tipping" from opposing pitchers. i.e. The smallest difference in a pitcher's delivery, imperceptible to you or I, would indicate what kind of pitch was coming. Writers often talked about how Alex Cora or Carlos Beltran or Jose Altuve were just remarkable "students of the game" and would figure it out and tell the rest of the lineup who would pounce upon it.
It's kinda funny to look back on, because I remember lots of people producing videos where they would supposedly point out what the Astros were picking up on, and it never seemed very clear to me no matter how much they slowed down the footage.
Another thing to note is that there's always fun baseball stats coming out. Kershaw getting no misses on his breaking balls would just be one of several interesting tidbits talked about the next day. It wouldn't really cause suspicion, as most people would just assume it indicates Kershaw was ineffective.
For example, in the the 2017 ALCS, Astros pitcher Lance McCullers finished off the feared Yankees lineup by throwing 24 straight curveballs. This is no suggestion that Lance McCullers cheated; it's just an example that there's always odd stuff to reflect upon the next day.