How do K-pop fans know how successful a group is for their company or not?

It's obviously glaring. BTS built an empire.

unless you are bts and sell millions of albums I don't think album sales make that much money for the company as people think considering all the expenses in recording costs, production, marketing , distribution. I think they make more money from royalties if anything.

touring, endorsements make more money

if they continue to support the group by giving them regular content and comebacks etc etc then it would be seen to be successful...

Quote from 2techs

K-pop fans usually say that album sales don't make the most money and that there's other ways of making more money like touring in japan, but how do they know that album sales don't matter for the company?

Everything matters. Just that touring is the biggest income. Doesn’t mean album sales are invalid