Go to INFJ or INFP? a closer look
Go down to Your Preferred Communication Style and read the link in its entirety.
While the link in question contrasts INFPs with INFJs, you can substitute "ISFP" for "INFP" in the text in question.
All types communicate in very distinctive styles, or forms. The forms by which INFJs communicate are directive; the forms by which IFPs communicate are informative.
This is a very well-known and researched feature of MBTI theory.
Once you become familiar with what these forms look like--and it isn't rocket science, it's just pattern recognition--you can easily read an sufficiently long interview with someone like George Harrison (or Bob Dylan or John Lennon) and go "Oh, this person prefers an informative communication style. Therefore, he can't be an INFJ."
When you start doing this it can be complex. Imagine using a flow-chart to determine whether an animal is a cat or a fox. When you get more experienced and adept at this it's really easy.