You can't detect it precisely unless you're the voter yourself, in which case you know pretty well if it's the content influencing your decision or not (eg if they're paying you to vote on their post, or if you're voting on their post because they're voting on yours in some stake weighted arrangement etc)
Externally, for now there are generally many hints as to whether the vote is bought or part of a circle jerk or some other dishonest scheme. Eg. If there are on chain links to vote buying.
Ultimately, if these things become less telegraphed, people will just have to get into the habit of downvoting whatever they believe is over rewarded more generally.