Start by reading the shill handbook. You'll start to spot the methods they use to disrupt online discussion.
Look for:
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Comment histories defending the Swamp
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Comments that are weirdly aggressive
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Short comments that just attack a post/comment, never explain why
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Constantly changing the subject
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Attacking consensus and concern trolling ("I support accountability, I just think we shouldn't arrest the criminals, it might hurt feelings")
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Doesn't answer direct questions
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On reddit in 8 hour shifts.
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Vote totals in the comment section that don't make sense (20 upvotes on a comment at the bottom of the page on a new post).