It really depends on the article and the company - for those that still exist (or at least existed into the 2000s) it's often possible to find info on their own website (or Internet Archive copies of it), or from listings on trade sites like Alibaba.
Sometimes (rarely) people manage to talk to staff from the companies themselves, sometimes there's stuff on Chinese forums, old magazine articles, copyright/trademark listings, credit listings in games, hidden text in ROMs.. Or sometimes just similarities between the games themselves (say, they use the same sound engine, or the same fonts) are enough to give away a connection.
I do try to reference things where possible, eg
this article has a few and other company-related ones do if you poke around, but I know a lot of the articles on there don't have anything.. so if there are some specific examples of articles or facts you want sources for, let me know and I'll see if I can dig anything up.
