Identifying pleasant PhD supervisors

I would note that when you ask the students, you need to read between the lines to a certain extent. Just as letters of recommendation are always positive, but supervisors will look out for what is not said in them, you should apply the same principles when you are assessing a supervisor.

Let me give an example: I was interviewing for a postdoc in the US, and the supervisor had the students and postdocs take me out for lunch. Over lunch I asked how hard people worked, and were they well looked after. The answers were the expected - they were well looked after, the boss had high expectations, but supported them well to meet these expectations, they worked long hours, but they wanted to. Then I asked them what they liked to do outside the lab. Silence. Apparently noone had any hobbies. Eventaully one of them ventured that they drummed in a band, but later came up to me and asked me not to share this with the supervisor. Perhaps it was a joke that went over my head. Perhaps it there were other things going on. But I wasn't going to take the risk of ending up somewhere where people didn't want their supervisor knowing they had a life outside work.

Asking current PhD students as Azor Ahai recommends is a good source. Another source I like are the PhD thesis of former students. These usually contain extensive acknowledgements and thankyous to the supervisor. Various information about the style of supervision can be gotten there. Note that only positive things are mentioned, so also look out for things supervisors are not thanked for.

In the US, typically, you would speak to current graduate students about their working style. In general, a cold email usually acceptable, but don't pester.

However, you might run into very different cultural expectations if you aren't yourself Japanese, so I would take their perspectives with a grain of salt.