Paulette L. answered 04/13/19
Rule of thumb would be to ALWAYS start with "vous" unless you're speaking to a child and then you automatically use "tu". Work colleagues tend to "se tutoyer" (call each other tu) immediately, so if you're in that situation, listen to what they're doing and follow suit. They may or not be using "vous" with the boss or higher-ups.
Sometimes you just have to ask if it's okay to "tutoye" someone -- e.g., an acquaintance with whom you've become friendly, or parent of a child in same school as yours. I would advise that being the foreigner (i.e., not French), your best bet is to "vousvoye" someone until s/he changes the tone and thus the grammar.