I used to think that harem was just when MC went around collecting girls so I would avoid them because most of the time it was just fan service and it just felt like MC was collecting pretty vases rather than actual compelling relationships with nuance and complexity. But then I think I've come to the recent realisation that harem can also just refer to a novel where there are just a lot of girls in love with MC and MC doesn't necessarily get with them. So now I'm questioning whether I should be more lenient and give some novels I would've normally shied away from a chance. But how would I judge that the harem part isn't going to be obnoxious fanservice? Because I once accidentally read a novel listed as harem and while there were girls in love with him, only three were mentioned by name and relevant and he only ended up with one of them, but I've also read stories where there are a lot more girls in love with MC, but they don't get listed as harem? So what should I expect from a novel listed as harem?
I always expect the worst, lots of girls/guys falling in love with the MC for dumb/weird reasons. Edit: You also need to consider the other tags like: Romance and Polygamy
Treat it as a warning that there'll be a Lot of girls in male-female ratio with male MC. I don't trust in reviews as most of them tend to overgeneralize a story and cant pick up the specific plot especially if they dont like the tag. These ruin the series from new reader and kill the translation all together. You can see these stuff in most harem stories review.
In the more traditional sense, a harem anime/manga/light novel is one where there are multiple explicit love interests of the same sex (i.e. love interests that have either confessed their feelings or that the MC realizes them); sometimes a love interest of a different sex may be thrown into the mix, but it's usually either as a comic relief or as a device to stir the dynamics. Whatever the MC does with the knowledge does not matter, although many isekai- and fantasy-themed stories choose the harem route. On the other hand, highschool-themed stories tend to choose a single girl's route (unless the highschool is not the main theme). To weed the novels that follow a harem route, I'd suggest to block the tag "Polygamy" instead (particularly if accompanied by "Proactive Protagonist" or "Threesome").