It can be difficult at times to distinguish between indirect and direct characterisation. However, direct characterisation can only ever occur in a story with a third person point of view, as it is when the writer tells us directly about what a character is like or what a person's motives are. Indirect characterisation is anything that allows us to infer something about a character, but it is not stated directly.
For example, in the story, we can tell through the setting and action that Jim and Della are poor. Consider how their apartment is described:
A furnished flat at $8 per week. It did not exactly beggar description, but it certainly had that word on the lookout for the mendicancy squad.
This is indirect characterisation - the author does not tell us directly that they are poor, but it is made clear indirectly through their setting.
However, later on, the author uses direct characterisation to tell us about the motives of Della and her purpose for saving money:
Tomorrow would be Christmas Day and she had only $1.87 to buy a present for Jim. Her Jim. Many a happy hour she had spent planning for something nice for him. Something fine and rare and sterling - something just a little bit near to being worthy of the honour of being owned by Jim.
Note here how the author directly tells us her motives for saving the money and how much she loves him. This is clearly an example of direct characterisation, as it directly reveals to us information about Della and her motives for trying to save money.
Hope this helps! Good luck finding more examples in this great short story.