I don't think boundaries are some mysterious, invisible concept; they're mostly just common sense and "politeness" of sorts. Do unto others, etc. . . will generally cover boundaries. People who are boundary crossers are those NOT paying attention or worrying about boundaries and who is "where."

If you cross another person's boundaries, they're in charge of telling you; you can't know where "their" boundaries are! No two people have the same boundaries/rules/worries. It's a little bit, as I imagine it, like with "personal space" -- you know how you feel when people are "near" you, how close you like someone to be or if you let people touch you or not, on the arm, shoulder, etc. Some people are less comfortable with touch than other people; boundaries are like that. But I don't "worry" a whole lot about crossing the other guy's boundaries because I expect there will be conversation and they'll let me know (or I'll sense) if I'm too close, over their lines, etc. It's a conversation/give-and-take but it's the people who are not paying attention, don't seem to "care" or think they have special priviledges because they've known you a long time or are your mother/husband or something :-) No one has special priviledges to you other than those YOU give special priviledges to! Likewise, the reverse is true.