Direct characterization is revealed through mainly exposition which prefaces the plot.
Indirect characterization is revealed through the action of the plot, mainly through dialogue.
Direct characterization with Wilson is problematic since Nick is not a primary witness to Wilson, either at the garage or before he murders Gatsby. Rather, Nick only hears it second-hand through Michaelis afterwards. We only get little pieces from Nick that border on direct characterization:
With an effort Wilson left the shade and support of the doorway and, breathing hard, unscrewed the cap of the tank. In the sunlight his face was green.
Like most of the women in the novel, Wilson is a flat, static character, so it is difficult to decide how Nick (or Fitzgerald) directly feels about him. Perhaps Nick, like Tom, doesn't really care about the lower classes.
An example of indirect characterization is in chapter 2 when Nick observes the conversation between Tom and Wilson:
“Hello, Wilson, old man,” said Tom, slapping him jovially on the shoulder. “How’s business?”
“I can’t complain,” answered Wilson unconvincingly. “When are you going to sell me that car?”
“Next week; I’ve got my man working on it now.”
“Works pretty slow, don’t he?”
“No, he doesn’t,” said Tom coldly. “And if you feel that way about it, maybe I’d better sell it somewhere else after all.”
“I don’t mean that,” explained Wilson quickly. “I just meant——”
We can tell that the upper-class Tom is holding the car, and his socio-economic status, over the lower-class Wilson. The dialogue reveals Wilson to be weak, not only in the conversation with Tom, but in his marriage and socio-economic status in society. It is ironic that Tom, who is cheating on his wife with Wilson's wife, will treat Wilson with such contempt.