How do I know?

Sinning is that which leads away from love.
We all sin, it is true, but the sinning that the Bible warns
against is sinning to do harm.  So if you decide to associate
with a doer of harmful deeds, you are showing approval
of the sinner sinning.  So it is not doctrine, but a WARNING
to be conscious of the danger of being an accomplice to
harmful deeds.
Got it?  A warning, not a rule.