Which details in the first two paragraphs of ‘‘The Night the Ghost Got In’’ by James Thurber tell you that this essay is based on actual events from the author's life?

The first relevant detail is the date given by the narrator for when the incident occurred—November 17th, 1915. This is crucial because one gets the impression that if this none of this had really happened, then the narrator wouldn't have made a note of the date.
In the second paragraph, the narrator elaborates further, telling us that the bizarre course of events began just after quarter past one in the morning. Again, this is another crucial detail which would appear to confirm the veracity of the story.
Later on in the second paragraph, the narrator adds yet more detail, mentioning the fact that his brother Roy went to Indianapolis and is expected home at any moment. That's who the narrator initially thinks might be responsible for the sounds emanating from the dining room downstairs.