How can you dispute a Chinese restaurant's charge practically, in Canada?

Next time, pay and report the restaurant to the Ontario's consumer protection board here.

You need to pay and you should pay with a card so that there is paper trail of the payment.

Make certain you take pictures of the menus, bill, and anything that could/should state or not if there is a cover charge or extra charge. Try to take the picture of the menus with your bill next to it, again, to show that it is the proper menu.

If there is a cover charge, it should be plainly written on the menu.

For better or worse, it is quite standard for dim sum/yum cha places worldwide to charge cover charges, often mildly disguised as fees for peanuts, napkins, tea, sauces, which are all brought to your table without asking. I've seen this in Hong Kong, Malaysia, Singapore, Australia, etc, which are notably all also places where cover charges in restaurants are not the norm.

The rationale is simple enough: eating dim sum requires an extraordinary array of little plates, sauces and utensils, and people tend to make an event of it, sitting around for much longer than (say) your average noodle place. So the cover charge is the restaurant's way of ensuring they make a modicum of profit from everybody at the table, even if they only nibble on one dumpling over two hours.

Yes, the restaurant was at fault for not making the charge clear in advance; but no, you should not feel like you, personally, were targeted in some kind of scam. And if your meal cost more than $40 or so, stiffing them on more than $6 of tips in response is really not cool -- it's the owner's policy, not the waitstaff's.

I'm Chinese and think this is really a culture difference. Not all Chinese restaurant have this kind of fee. Only Guang Dong (?????) and Hong Kong restaurants have this kind of fee.
As a Chinese, the first time I went to Guangdong, I was surprised about this fee too. There, it's called ????- something like you go to restaurant and take a seat to drink a pot of tea. You will be asked to pay this little amount, no matter if you drink or not. The first time this happened to me I was surprised: "Why should I pay even I don't drink tea?". Later, I know there is a history here at Guang Dong.

In Guangdong province and HongKong people like to drink tea at a restaurant and have a chat for the whole day. They call this "???" (drink morning tea?).
Just like Westerners like to go to a bar and drink something and have social talk. Often, they only ask for a pot of tea, because what they eat and drink is not important, talking and friends are. And as a convention, the tea pot should be filled when it's empty. They may even order nothing.

But if everyone goes to a restaurant and orders nothing or only a pot of tea, and occupy the restaurant for a long time, the restaurant will go bankrupt. So, it's just common at Guangdong/Hongkong.

In fact, there is some discussion about this 'table cover fee' or 'tea position fee' in China too. Some people think this is unreasonable. But most Guangdong restaurants will charge for this to make themselves more authentic.

Pay the bill, then double-check the menu and the entrance for signs informing you about the "sauce fee". If you don't find anything, inform the restaurant in writing that you disagree with the charge, joining a copy of the bill and the evidence you would collect while double-checking, like photos of the menu and the entrance. Give the restaurant enough time to deal with your complaint (1 month would be considered enough almost universally), and ask them clearly to reimburse the charge you disagree with.

Chances are that the restaurant will decide to issue a refund. Or perhaps they will reply and provide convincing evidence justifying the charge. If not, you have solid paperwork to file a complaint with customer protection authorities.

This is as far as you can go without going over the budget of $6 which you're trying to get back.

Why not simple decline to pay (that part of the bill)? Whom would they report you to?

If they call the police, you can make your argument there, and you would potentially win. And whom else would they report you to?

Here in New Zealand, I've not seen cover charges like this in a restaurant - including at Chinese Yum Cha restaurants and similar. Also note that, like Australia, we don't do tipping here either (businesses pay waitstaff properly).

If I saw something like this, as an unannounced final charge, I would question it; and if I received an unsatisfactory answer, then it would be a negative review online, giving full details. Restaurants (and accommodation) here take their online reviews very seriously (as we're a tourist country) and too many people complaining can end a business.