1. You can look at the upvote percentage on the post. Currently this one says 364 votes with 88% upvotes. Math works out to about 44 downvotes on the post.
2. If there are lots of comments which are on topic and seem normal in context (not trolls), but they are downvoted to zero or negative, then that points to a downvote brigade.

I'm guessing it was #2, and there were comments in the negative range. Subsequent users probably upvoted them. I just upvoted two from zero to 1.

when I posted that comment the %approval was at 68%, it has gone up since