Less often than you'd think. Maybe for the biggest publishers, certainly for the big publishers who otherwise go after videos and take them down, but disclosure is actually pretty good for the most part. There was big mess kicked up in 2014 about Microsoft not requiring disclosure that the video was paid for and it scared everybody into doing it right. XCOM2 had ample pre-release coverage from outlets of all sizes and most of that wasn't paid.
Plus like with anything else, you can just find the people who have credibility and are honest about whatever agreement there was. Paradox Entertainment has flown people to events for all of their games and allowed preview coverage of them, but they don't actually pay for the videos. Just like with old journalism as long as you have people telling you that up-front there's no real issue with it.