I think part of the problem is that there is no real interest in collecting far eastern glass at the moment. If someone picks up a vase at a fair or wherever, and sees a label on the bottom saying "made in China", then they are probably more likely to put it back down than add it to their collection. Then there is the language barrier. I've often used Google Translate to do searches in Swedish etc when researching, but trying to search in Chinese or Japanese has never really lead anywhere for me.
Having said that, there is of course the China + Taiwan section of this forum
here, and I've delved a little bit into Japanese glass
here, but it's only the tip of the iceberg.
All I was curious about was why/how people are so sure regarding the alleged Chinese/Japanese pieces without very much ready information.
A lot of it is speculation, like M says, you just get a feel for some of the characteristics, such as base and rim finishes, how the glass is cased etc, which might leads us to say "probably" Chinese. I don't think anyone is "so sure" though, without evidence of labelled pieces.