Thank you for your question! Reading between the lines, I assume your concern is that when people figure out that you are anglophone, they will switch to English out of convenience.
Firstly, let me opine that French-speaking Quebecers are on the whole extremely gracious towards international visitors who make efforts to learn and use their language. You will never offend anyone here by speaking with a grating accent or by making grammar mistakes.
However, it’s important to remember that not everyone has the time, ability and/or interest in giving you a language lesson, so in a social context, take it as a compliment when somebody switches out of their language to pursue a meaningful conversation with you. Also, don’t forget that Quebecers are sometimes looking for opportunities to practice their English! Moreover, in Montreal, one in two people has English as either their first language or their first Canadian language. In a business context, it’s different: you have the right by law to be served in French.
As your French gets better, you will find people “switch-out” less and less. You will also make friends who prefer speaking French with you. My advice therefore is to jump head first into this fabulous language, and when you are capable, persevere with it even if you get the occasional response in English. Finally, you will find that the various regions of Quebec handle this issue differently - in general, the further you get from Montreal towards the North, the less likely people are to speak English and/or switch out of French. I hope this helps!