That's a very good question. When I was a younger student in Highschool I used to wonder how my teacher knew what arrow to use. To tell you the truth, I don't have a good answer, but I'll try. It really comes from experience. In general, any resonance structure of solvation of a compound will have the
If you have reagents A + B turning into C + D then it'll probably be
I keep thinking about it, and it is a really good basic question, but I just don't have an answer. At this point I just intuitavley know which arrows to put up but can't explain it, and I usually pride myself in being able to explain chemistry to people.
In reality though,
denotes the more realistic happenings of the chemical reaction, because even if JUST ONE SINGLE MOLECULES out of billinos upon billions reacts to form product A, then you can use the equilibrium arrow, but in that case people usually draw a longer right or left arrow then the one beneath it, to denote that ONE SIDE IS FAVORED which is sometimes the case.