When does a Cisco router generate a source quench?

A Cisco router generates a source quench only if it does not have the buffer space needed to queue the packet.
If the router can’t queue the routed packet onto the output interface’s queue, it generates a source quench and
registers an output drop against the output interface. If the router isn’t congested, it won’t generate a source quench.

show ip traffic – Output for source quenches sent.
show interface – See if there are any drops. If there are none, then you should not see any source quench.