Re: How do particles know where photons come from?

Excellent question for which there is no common-sense answer.  As you 
advance in your studies in physics, you will find many more of these 
occurrences, but please don't allow this to discourage you;  it's fun.

To answer your question, consider the analogy of "how does a raindrop know 
that it's going to strike you on the head?"  It doesn't, of course, any 
more than an electron "knows" when/whether a photon is going to strike it. 
Further, it does not matter to the electron whether the photon came from a 
positron or another electron or from some other source.  It only knows 
that a photon has struck it.  The way you approach this problem 
quantitively is to invoke the laws of probability in an arcane, but 
fascinating discipline known as "statistical mechanics", usually 
approached in the last years of undergraduate study and certainly in grad 
school.  What you do is calculate the probability that one of a group of 
photons will strike a given electron and kick its energy level up.  BTW, 
the same thing applies to particle attraction/repulsion, where the 
attraction occurs thru the exchange of a particle called a 'graviton'.

To help (and also complicate matters) there are the uncertainty principles 
of quantum mechanics, which I surmise you are studying right now.  They 
are contrasensical, but have never been disproven.

I don't want to get into the mathematics now, but a couple of good 
references are:
1.  BH Mahan, Quantum Mechanics, ~1972.  Paperback with very good english 
explanations along with careful, reasonable mathematics.
2.  More advanced:  SM Blinder, Advanced Physical Chemistry.  Lots of 
math, but excellent english explanations.  Excellent coverage of quantum 
and statistical mechanics, along with some thermodynamics if you like.

Good Luck!