The same way you check what you buy from an online store: by the number and content of the negative reviews or downvotes. But they don't exist here because inclusivity is wanted more than quality.
Like counts, be it likes alone or Reddit-style with downvotes, is the wrong mechanic for community-generated content, as it incentivises posting something "for a quick like". As explained in my other reply, Slashdot has a really healthy, decade-old community without having like counts. Posts are ranked by comment count and if there is something really bad, users can report it. It's that easy and it works.
That's how I feel about it, it's essentially the way stuff is incentivized on 4chan, and we all know that community isn't really know for its high quality, friendly discourse...