What is the ping and traceroute command option to use in order to check the connectivity between 2 machines (machine A and machine B) using my machine (machine C) ? How do I specify the source machine IP? ping -S machineAIP machineBIP or ping -I machineAIP machineBIP don't seem to work from my debian 8 machine.
ping allows you to set intermediate hosts, I think you can only do this if the intermediate hosts are willing to accept traffic intended for the destination (i.e., they behave as gateways). See https://superuser.com/questions/311849/how-can-i-ping-via-an-alternate-gateway
In this situation, I'd just use SSH:
ssh machineA ping machineB