I don't know, as I said, "I would expect they've chosen that option, they have a brand to protect." They have a brand so good in this domain they've be absolutely insane to commingle.
But, no, we can't tell if a vendor commingles, we can only infer it in special cases like Anker as manyxcxi noted, or pick out vendors with high ratings and otherwise cross our fingers.
It's a colossal mess, and nowadays, if I'm not buying used books (where I go with < 95% rating and Very Good quality), I find a BS detector honed for decades, literally about 4 and half, is the single most useful tool in navigating their swamps. Which for me confirms they indeed have big problems, and that's not even counting their search engine, which as others have noted is bad; for me, by far the worst I regularly use, and like others I often use Google's instead.
All this implies to me that if AWS wasn't such a money machine, Amazon's stock holders ought to be very nervous. The company really need to get serious about these problems.
edit: here is some discussion. FBA vendors appear to have control:
https://www.amazon.com/gp/help/customer/display.html/ref=hel...
But I still don't think the amazon customer can see if an fba vendor allows commingling.