What they want to do? This request seems like asking for a roll so that it's acceptable for the PC to think something. Not my responsibility as a DM to determine or adjust what a PC thinks.
This is my fundamental issue with Insight as it's usually discussed. It's usually discussed in a manner of "is there a clue so that my PC is justified in having this opinion of the fiction?" The response is to use Insight to find this clue. The problem this creates was identified in the last thread on this topic -- what happens if the NPC is telling the truth and the PC fails the check? This gets into the GM presenting fiction specifically to mislead the players, only it's either known (the PC sees the roll) or always hidden (the GM always makes secret rolls, so the player isn't really getting any reliable information to begin with). Instead, I use Insight (and all skills) as active resolution mechanics. Insight, for me, is used when trying to find out information from an NPC via social interaction. This requires interaction with the NPC and isn't a check made against what the NPC has said to determine truthiness.
For example, using the OP example, the PCs may try to find out more about the owner, eliciting Insight checks to extract information (against the NPC's attitude DC). When they feel like confronting the owner, their approach may be trying to find out information from the NPC leveraging clues they've found, which would be an Insight check.
Mabye Bob's character starts by trying to intimidate the owner into telling the truth by looming and yelling that the owner's lying. Susan's character takes the opportunity of the owner's being distracted to look around the store for anything about the owner she can leverage if the intimidate fails. Which it does, and the shopkeep backs away from Bob's character sticking to his story and threatening to call the watch. Susan's character's perception check succeeds and she sees a child's drawings pinned under the counter and a few toys. Susan's character steps in between the shopkeeper and Bob's character and tries to defuse the situation by shooing Bob's character away and apologizing to the owner for the bad behavior (persuasion check success), then starting up a talk with the owner about their child, trying to improve attitude (Insight check with advantage for having good info, success). Now that the owner is placated and talking Susan tries to leverage concern for the child into finding out more information about the murder last night by promising protection for the owner's family and silence on where any information comes from. This is another Insight check (trying to elicit information), success meaning the owner talks and failure causing him to shut up and demand they characters leave the store.
But, at no point would I let a player ask for a check to determine if their character thinks something or not. The players are either suspicious, or they aren't. I also follow an 'active PC' mode of play where NPCs don't usually roll against PC outside of combat. NPCs do things, and players act. If an NPC is being intentionally deceptive, I'll use their passive Deception as the DC for any elicitation checks against them rather than the usual attitude checks. A successful Insight check uncovers the deception, but the player has to be engaging the NPC with a goal and approach (as above) that would result in asking for a check. Failure means the deception would be repeated. The owner just not being willing to offer information isn't an active deception, in my book, just recalcitrance. The owner being the murderer, on the other hand, would likely be an active deception.