There is currently no officially supported way to check discussion posts for plagiarism (or AI-generation). Plagiarism checking for discussions is a feature that has been highly requested by Instructure Community users for a long time, but has not been developed. I have not seen an exact reason for this, but I think it may be as-much on the plagiarism detection providers side as it is on Instructure's. While assignments can be run through plagiarism detection systems currently, the volume of discussion posts (at least at my institution) is higher than assignments, and the discussion posts are often shorter or less robust. I can imagine that putting all of those posts into a plagiarism detection database may overwhelm those systems, which could end up marking many more things as plagiarism when they may in fact bust be commonly used phrases in discussions. This isn't an official reason by any means, just my musings as a higher-ed Canvas admin and computer science graduate.
Some people suggest copy/pasting an entire discussion thread into a plagiarism detection system, and while that may provide some results, you'll want to be careful when doing that. First, you probably don't want that added into the database under your name. Second, you may need to check with your school to make sure it's okay to submit student work into another system without their express consent. You'll also only receive one score this way, so you'll need to look through the report pretty closely to see which posts (if any) may have more similarity matches than others. A few things to think about there.
I hope this post helps in some way, even though it may not be exactly what you wanted to hear.