How do I know which action would fall under maris ayin and which would not?
According to Iggros Moshe, maris ayin applies when “Someone thinks that I violated something, or he thinks that I inappropriately ate something non-kosher. However, it does not include doing something permitted that people may mistakenly think is forbidden due to the fact that they do not know the halachah.” A permitted act, which looks like a forbidden act, is a case of maris ayin. Where maris ayin applies, it is forbidden even in private.
For example: It is forbidden to drink almond milk after eating meat, unless almonds are placed next to the beverage, indicating that the while liquid is not cow’s milk; one may not enter a non-kosher restaurant, unless there is a great need for this (such as using the bathroom); one may not drink human blood, unless the cut is in the mouth itself (for fear people will think it is animal blood).
Sources: Iggros Moshe, Orach Chaim 1:96, 2:40, 4:82; Minchas Shlomo 2-3:53; concerning the example of almond milk, see Shulchan Aruch, Yoreh De’ah 87:3, Chochmas Adam 40:3, Aruch Hashulchan 16.