I'll give some answers, then question your assumptions.
Using wireshark from both endpoints and matching traffic to see what's seen on one side but not the other is the only way to know for sure what each node sent and received. This is not usually very practical.
You can also ask query your switch for L2 and maybe L3 statistics on the port(s) in question, and maybe things like buffer problems or CPU/memory issues on the switch. If the switch is dropping packets because it's too busy or flooded, you'll want to know that.
Now, what I'd like to know is, why are you certain that the problem is the network? To give the best help, we'll need a detailed description of the actual symptoms you're observing, and what troubleshooting you've done. Many times, the problem is not the network.