Speaking of Barry being able to read Leonard Snart scarily well, imagine that Barry notices how Snart reacts to people Barry knows, and files that away in his opinions of that person
Barry is in the cortex with someone he finds kind of annoying but harmless, Snart walks in for Definitely-Not-Flirting-While-Doing-Business Reasons, and starts lowkey acting like this guy is an Immediate Threat that he's trying to act cool around to not set him off. And Barry immediately knocks like 5 points off of the "how much do i trust this guy" ranking. Subtly starts telling him less things, looks into what he's doing on the downlow, treats him as potentially suspicious
And turns out... The guy does have bad intentions
Leonard comes in and meets some of Barry's new friends/coworkers, and clearly seems to mark one of them as more trustworthy/less of a threat than everyone else. And when Barry needs help or someone to confide in, he goes to that person over everyone else
Barry comes in to Saints and Sinners and sees a regular he's run into before, but hasn't seen interact with Snart. He thought the guy might be pretty cool. But today he walks into them talking, and Immediately sees that Snart is closed off, jumpy, does NOT like that guy in his space. And Barry immediately takes cues off of that, turns the guy down when he later asks to have drinks together next time Barry drops by Saints
If Snart seems unnerved by someone Barry knows, Barry trusts his instincts. Even if Leonard never says anything to Barry out loud, he still takes note of that. If Snart likes someone, or just thinks they're reliable, Barry will trust that instinct.
Even if Snart doesn't say his feelings out loud, Barry trusts him so innately that he will listen to Snart's instincts when it's clear he's feeling a type of way