“It's very dry.”

Your brother tells jokes without laughing, so you can never tell whether he's joking. You're talking about him with your daughter, and she says, "He has a weird sense of humor." You agree, and you want to describe his sense of humor more specifically. You say:

It's very dry.

a dry sense of humor

If someone has a "dry" sense of humor, it means that they make jokes or say funny things quietly, without directly showing that they're joking. A person with a dry sense of humor usually doesn't smile or change their voice much when they're joking. It's almost like they're hiding the joke so that only smart people and careful listeners will get it. British people are often thought of as having a "dry" sense of humor. Here are some other ways that people can be funny:

• If you have a dirty sense of humor, it means that you tell jokes about sex.
• Absurd humor is when people do things that are ridiculous and don't make sense.
• Slapstick humor is something that you mostly see only on TV, movies, and in plays. This is when someone gets hurt, drops things, falls down, etc. It's also called physical humor.

“I can never tell whether he's joking.”

You have an uncle who has a strange sense of humor.

He tells jokes, but his face always stays serious. You get confused by this. Now you're talking about your uncle's personality with your mother, and you say:

I can never tell whether he's joking.

I can't tell whether (clause)

In this case, "tell" doesn't mean "say". It means "decide" or "figure out". The phrase "I can't tell ___" is used to talk about something you're not sure about:

I can't tell whether it's finished or not. I can't tell whether she's being shy or just doesn't want to talk to me.

The examples above use "whether", but you can also use other question words like "how", "what", "who", and so on:

I couldn't tell what they were saying. For the life of me, I can not tell how he did it.

(someone) is joking

When someone "is joking", it means that they're saying something that's not true, in order to be funny:

A: You're quitting? Are you serious? B: No, I was just joking.

You probably also know that "jokes" are little funny stories or sayings that you tell people. When you're doing this, it's called "telling" a joke:

He always tries to tell jokes, but he usually messes them up.