When the narrator wasn't around Emily how did he get his information?

When the narrator was not directly in Emily's presence (which, because she isolated herself for so long was much of the time) he gathered his information about her from other people. Even in his opening description of her, he sites a tale invented by Colonol Sartoris:

Not that Miss Emily would have accepted charity. Colonel Sartoris invented an involved tale to the effect that Miss Emily's father had loaned money to the town, which the town, as a matter of business, preferred this way of repaying. Only a man of Colonel Sartoris' generation and thought could have invented it, and only a woman could have believed it.

He sites, too, the tales told by the ladies of the town who complained about the smell emanating from her home. He indicates that they were not surprised about it because her house was being kept by a negro man and, in their eyes, a man could not possibly be expected to keep a kitchen clean.

In fact, a close reading of the text allows you to note that the majority of the narrator's information comes from an undefined "they" - meaning various people of the town ranging from government officials to gossipy women. He associates himself as a member of the town among a group of people simply known as "we" such as in this statement:

We had long thought of them as a tableau, Miss Emily a slender figure in white in the background, her father a spraddled silhouette in the foreground, his back to her and clutching a horsewhip, the two of them framed by the back-flung front door. So when she got to be thirty and was still single, we were not pleased exactly, but vindicated; even with insanity in the family she wouldn't have turned down all of her chances if they had really materialized.

Note the repeated use of we in the narrator's observations. It is evidence gathered from the collective consciousness of a town that has built up a legend around this woman and who do not become privy to the truth until after her death. If you continue a detailed examination of the piece, this becomes evident and you are afforded with many other examples of second-hand information about Emily's actions.