My experience is mainly on Amazon co uk - but a lot of indie books (or rather - the subset of indie books from authors who still try to self promote on forums), have very suspicious reviews.
There are a lot of self reviews - sometimes the author is silly enough to use their own name, sometimes thinly veiled, often they are silly enough to use a different name, but post 'read my book ...'
After those - it is the friends and family - these are mainly obvious by the weird claims, the review date pretty much within a few days of book publication date, and being the only review (or often there is a cluster of reviews just around that one). The weird reviews are often - 'I don't normally read <insert genre> but I found this one and...'
There seem to be very suspicious author review groupings - maybe these authors did all come from the same writing group or creative writing course or something - but they must be a bit silly to think that reviewing each others books constantly, and consistently high, isn't going to look suspicious.
And the people who give 5* to disparate things in a short period, which look like paid for reviews.
I assume Amazon's removals are pretty tightly controlled - when I have notified them of dodgy reviews they have only ever done anything if there is substantial proof. Even very strong circumstantial evidence tends to get ignored...
And ultimately - I'm guessing they make a fair bit off indies, and think they can probably make more if they could have mechanisms to actually help identify good books (especially as they get in to publishing...)
There have been a number of articles over the past two years about them investing in fake review investigation software and techniques. Presumably they have started using them. Bravo is what I say.