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This comic follows on from the Previous comic which will almost certainly provide context.
The spell Konsi's talking about is "Fortune's Favour" - it's level 2, consumes a 100gp pearl to cast, and lets you re-roll a single D20 within the next hour. It's in Explorer's Guide to Wildmount.
It's been a little while due to drawtober, and my website is (mostly) functional again, so if you want to read all the Konsi comics from the beginning, you can do so on my website at this link. Please be advised that these posts are presented in reverse chronological order, so start at the end and work backwards.
Why don't they just cast Zone of Truth and ask her if she cheated?
Addis explained in an earlier strip - the people making the accusations are not interested in whether or not the accusations are true, they're interested in manufacturing a scandal.
When people with an agenda are pushing conspiracy theories, there is no test or proof you can administer that will satisfy them. They will just move the goalposts. "She found a magic way to cheat the zone of truth, that's how awful she is." "She's so corrupt she doesn't consider what she did to be cheating." "The church cheated for her and she didn't notice". "The person casting Zone of Truth is part of the conspiracy."