After the first of Gatsby's parties to which they are invited, it is clear that Tom begins to feel threatened by Gatsby and his lavish wealth. He makes clear his intention towards the end of Chapter Six to look into Gatbsy and how he so quickly gained such wealth and prestige. In Chapter Seven, it is clear that he now has that information, which he uses as part of their argument over Daisy whilst in the hotel during the blisteringly hot day. Tom says to Gatsby:
"I've made a little investigation into your affairs--and I'll carry it further tomorrow."
Thus it is that the investigator he has paid has told him precisely how Gatsby has made his wealth, and it has been by working with Meyer Wolfsheim and buying up lots of drug-stores where they sell grain alcohol over the counter.