The dictionary definition of a dictator is that it is someone with absolute power, especially a leader in government who has absolute and unrestricted authority. This implies that there is no right to vote for citizens and that the leader is unrestrained. His word is his command and he has overriding power. His authority is therefore unquestionable.
In the novel, the following quote would probably best describe the fact that Napoleon had become a dictator:
... Napoleon, who had remained on his feet, intimated that he too had a few words to say.
Like all of Napoleon's speeches, it was short and to the point...
... This farm which he had the honour to control, he added, was a co-operative enterprise. The title-deeds, which were in his own possession, were owned by the pigs jointly.
He did not believe, he said, that any of the old suspicions still lingered, but certain changes had been made recently in the routine of the farm which should have the effect of promoting confidence still further. Hitherto the animals on the farm had had a rather foolish custom of addressing one another as "Comrade." This was to be suppressed. There had also been a very strange custom, whose origin was unknown, of marching every Sunday morning past a boar's skull which was nailed to a post in the garden. This, too, would be suppressed, and the skull had already been buried. His visitors might have observed, too, the green flag which flew from the masthead. If so, they would perhaps have noted that the white hoof and horn with which it had previously been marked had now been removed. It would be a plain green flag from now onwards.
... that the name "Animal Farm" had been abolished. Henceforward the farm was to be known as "The Manor Farm"--which, he believed, was its correct and original name.
In this speech, Napoleon is addressing a meeting of humans and pigs who had come together on the farm to celebrate its success.
It is pertinently clear from the speech that Napoleon has achieved ultimate power. He unambiguously states that he controls the farm. Added to this, none of the changes which he mentions have been introduced after consultation with the other animals. He alone decided on them. Furthermore, he had arbitrarily decided that all the old traditions would not only be abolished, but would also be suppressed.
There is a slight distinction in the connotations of these two terms. 'Suppressed' suggests that the activities of the other animals would be monitored and, if they should undertake any of the old habits, they would, in some or other way, be punished to put an end to such practice.
It is also obvious that Napoleon, as supreme leader, has alone decided to change the flag and to revert to the farm's prior name. He did not even bother to ask the other animals' opinions about this. One character making such subjective decisions is truly the mark of a dictator.
Since there are so many editions of the novel, an accurate page number cannot be provided, but an easy reference is that this passage is found almost at the end of chapter 10.