How does Old Major estabilish that he is an authority ?

The text does not explicitly say why Old Major was respected on the farm.  However, there are a few details that offer some clues. 

First, when Old Major is introduced, he is described as a prize Middle White boar.  So, in some ways when he was younger, he was prized by Mr. Jones and presumably others. The second paragraph of the books has these words:

Word had gone round during the day that old Major, the prize Middle White boar, had had a strange dream on the previous night and wished to communicate it to the other animals.

Second, the fact that he is called "Old," also gives him some gravity.  He has been around long enough to understand the farm and what happens to the animals on the farm.  So, when he speaks, he speaks with knowledge, and the other animals listen.  When we look at this speech, he is cogent, he gives examples, and calls for action - no wonder the animals were willing to loose an hour a sleep to listen to him.