In Chapter Six, how does Tom sense immediately that Gatsby is a bootlegger, selling booze to speakeasies?
To answer this question, take a look at what Tom says about Gatsby when asked why he thinks he is a bootlegger: "A lot of these newly rich people are just big bootleggers, you know."
In other words, Tom's suspicion is based on his prejudices about people who come from a new money background and not old money, like his and Daisy's family. In Tom's mind, new money people are only capable of making money from illegal or immoral activities since they lack the power and prestige of the old money class.
Daisy tries to convince Tom that Gatsby made his money in an honest way, by owning a large number of drug-stores. Tom, however, will not be persuaded. In fact, Daisy's efforts to defend Gatsby only make Tom more determined to find out the truth. Tom's prejudices are, therefore, hardened by Daisy's comments, and Gatsby has no chance of convincing Tom otherwise.
When Nick asks Tom where he had heard that Gatsby is a bootlegger, Tom says “I didn’t hear it. I imagined it. A lot of these newly rich people are just big bootleggers” (114). Gatsby is very visibly successful: he displays an ostentatious opulence in his parties, his house, and even the way he dresses. Tom distrusts anyone who could have so much money and show it as Gatsby does. It would annoy Tom that anyone could “get rich quick,” and he would immediately attribute doing so to talents that are unsavory or at least denigrate the talents any other many might evince. That is why he says “just,” indicating success as a bootlegger does not count as real success. Tom is prejudiced, narrow minded, arrogant, and violent as well, quick to criticize others, especially if it makes him feel more powerful, more manly.
Tom is just like everyone on the West Egg if you are not from the elite, old money category then you have no money and status. Around this time people were making big bucks on Boot legging liquor and the way Gatsby rose up so quickly in wealth fits well with being able to efficiently bootleg liquor. Then and again Tom was skeptical of Gatsby riches and the way he flaunted it he would use anything that destroy Gatsby's reputation.