How do I figure out the contents of doujinshi? I have been to Japan and saw all these doujinshi manga which all look really interesting just by their cover. But how do Japanese people actually figure out the content of these books? There is no plot written anywhere, and you generally cannot open them.

For example, I have been reading 孤独のグルメ (Kodoku no Gurume) and I saw doujinshi based on it. Should I think "the manga was good so let's try this version"? Or is there somewhere to look at to find out the content?

Does it work only by word-of-mouth or are there any resources on the internet we can check to have more information on the different authors and their doujinshi?
It might be a simple matter of H-service, so what matters the most is the art. Something like storyline is trivial in most of the H-works in Japan as I would say. That's why, the business focuses only on gaining money.