I like this approach of Persuading them to offer truth or else comparing their statement with what you know. It is much more truthful to not only reality but also most fiction that isn’t predicated specifically on supernatural lie-detector characters.
In reality, studies have shown that people who believe they can discern lies (just from hearing them, rather than investigating) are no better than chance and sometimes even worse.
The way to spot a lie is to find contradictions with known fact. You can easily suspect someone has lied, of course, but you can’t really know. That’s just not a thing humans (or presumably mice) can actually do.