hard to say. we get this question almost on a daily basis. the problem is that there are millions upon millions of apps each with their own notification icons. so unless it is something like Facebook or youtube, it will be hard to determine what it is. you might get lucky and someone here might know it, but do not hold your breath on it.
the easiest way to know is to look into your phone's notifications to find out. but 99.9% of the time, the phone is owned by someone else. you can always just ask the person, but that is not as easy as it sounds i guess.