Simply 'separate the good from the bad' is perfectly good. You could use: separate the wheat from the chaff, if you want to appear to be, say, erudite.
I wouldn't call it
that erudite. Like many of our most expressive idioms, it's Biblical. But it's certainly in colloquial usage - in, for example, the Crosby, Stills, Nash and Young song 'Almost Cut My Hair'. Another possibility, also Biblical, is 'separate the sheep from the goats'.
Another one, which doesn't mean quite the same, is 'separate the men from the boys' (but that means making distinctions on grounds of age/ability/maturity/experience). I've also heard 'separate the gold from the dross', but that is a bit mannered.
b