When we consider process like
$e^- e^- \to e^- e^-$
in QED, we see that from exchanges of one photon (tree-level diagrams) one can obtain Coulomb's law, while loop-diagrams give quantum corrections to this law. I know that virtual particles, which appear in loop-diagrams, are not observable. But real particles can be observed.
So, I wonder, are such photons that contribute to classical Coulomb's law (tree-level diagrams) observed? For example, we can increase distance between two electrons, so that virtual photon becomes almost real.
Or is there any way to experimentally show that electromagnetic forces are caused by discrete particles - photons?