I have very occasionally tried to contact the host to find out what the problem is, but didn't find they replied very often. So now, I try to make a judgement call, if the guest has lots of great reviews and one outlier, or even one great review and one bad one, I check the reviews the host has both received and given. In some circumstances, it was clear that the host was the one who was slightly unhinged, often leaving similar bad reviews for multiple guests (all of whom had good reviews from other hosts).
One shouldn't have to do so much homework, but there are bad hosts as well as bad guests. A host could be stellar in many ways but then extremely intolerant about very tiny things. It's not often that clear cut.
In general though, I do what @Ann72 does and ask the guest directly. I tell them they have low star ratings (usually they have no idea about that) or mention that one of their reviews wasn't that great and ask them what happned. Usually there is an explanation and you can take it or leave it.
A recent guest had loads of glowing reviews and one really bad one. A couple of days after he arrived I asked him about it and he explained exactly what had happened. I kind of get why the host wasn't totally happy, but it does seem like they over reacted somewhat. Anyway, this guest turned out to be fine and I did not have problems with him like the critical host complained about.
It's tricky really as the review process allows for far too much personal judgement and what is okay for one host might not be for another.