Sometimes it might be worth starting with a question to find out if it is a problem... something like "How often does someone who answers a question downvote the other answers to that question?" By comparing whether those competing answerer's downvotes are the only downvote the answer receives, you might be able to guess at whether it was a valid or invalid vote... for example, if someone downvotes an answer that gets five upvotes and no other downvotes, I'd wonder... but if an answer gets five downvotes and no upvotes, that's pretty clearly not tactical. —
Catija 1 min ago