One way you can tell if a Chinese restaurant is good is if there are Chinese people eating there. Restaurant Ruby Rouge caters almost exclusively to Chinese-Canadians; I'm always one of the few non-Chinese people there when I go. I can't judge whether the food is authentic, but it's very good.
I love this place, and I come here every time I'm in Montreal. The atmosphere is great -- formal Chinese dining room -- the wait staff are friendly, the service is good, and the food is excellent.
If you're in a group and feel adventurous, don't bother looking at the menu -- just tell your waiter what things you like, and let them bring you whatever they want. I've discovered many amazing dishes there this way.
They're a little pricey, but worth it, because you get good quality Chinese food. And don't do takeout -- the dining room is a fun part of the experience.