_now

1 As a discourse marker, now is commonly used in a mocking tone at the end of a rhetorical question to sarcastically echo someone's intentions, claims, threats, etc. that you find ridiculous or even insulting. The nuance at work in this type of expression is «if that's what {someone} thinks, they're very mistaken!».

On this site, the notation _ stands for a low intonation - the kind you hear at the end of a statement.

____________ 

Note that Oh can be added at the beginning to express feigned surprise and make your question sound even more sarcastic. 

He says he can do your job a million times better than you.— Oh can he now! 
An even more sarcastic version of this type of question is formed on the pattern: Oh he can, can he! Oh you do, do you! etc.
This type of expression with unstressed now always echoes the proposition in the previous statement with a modal verb. It throws the proposition back in someone's face and is very sarcastic: He says he can do your job a million times better than you.— Can he now! He says he'll tell your boss.— Will he now. He says you should hire an attorney.— Should I now! If there is no modal in the previous statement — as in this snippet — then do is used.He intends to borrow your car.— Does he now!
_______________⦿ I'm good | I don't need anything for the moment⦿ I take it that's a no? | I assume (from your reaction, expression, etc.) that your answer is "no"?⦿ Who goes to a concert? | Here: What kind of person thinks of going to a concert at a time like this?

Father: Hey, I'm on my way home. You need anything from the store?

Mother: No, I'm good. But I have to warn you... 

Father: What?!

Mother: Alex is going to ask you to let him go to a  concert in Miami with his friends this weekend.

Father: I see. And how exactly does he plan to get there? 

Mother: Well, he intends to borrow your car. 

Father: Does he now!

Mother: So I take it that's a no?

Father: Absolutely. He's not insured for my car, number one, and number two, there's a pandemic! Who goes to a concert?!

2 This type of expression is used when the speaker questions someone's ability to do something in a mocking way, like the very idea of something is so preposterous that it makes them laugh. 

Kevin: Ronnie says he can beat anyone in this office at tennis. 

Danny: Oh can he now! Does he realize that I'm a semi-professional tennis player?

Kevin: Yep, he still says you're no match for him.

Danny: Does he! I almost don't want to play him; it'd be like shooting fish in a barrel. 

Kevin: Who knows? Maybe he has hidden talents.

Danny: Hidden talents? He's about 75 pounds overweight! He can barely make it up the stairs in the morning. Oh well, it's his funeral!

⦿ you're no match for him | you don't have the skills to compete with him⦿ almost don't want to | almost feel guilty about doing something⦿ like shooting fish in a barrel | something so easy to achieve or win that it's almost too easy. It compares trying to catch fish in the open water with just shooting them in a barrel of water.⦿ it's his funeral | he'll never succeed/win, but if he wants to try, let him! Stress both his and funeral.
Note that in response to the proposition He still says you're no match for him, Danny responds simply Does he without now. Here, his tone is enough to express his sarcasm. The use of now in this type of expression is actually optional as long as the tone is correct.