OK, you asked "most accurate".
That would be with an experienced machinist, using his own tools, the proper tools for the job, that he is used to using and has used day in and day out for years in a controlled environment with the same temperature and humidity conditions day in and day out.

If you are talking about a person who builds one motor a year in his garage, then plastigage works pretty well. I'll put it as simply as I can, this stuff ain't rocket surgery.