When would over subscription actually become an issue that I can show management via a show command on the switch.
When I look at the cards I see only output drops on the occupied ports. Utilization is low and there are no other errors yet users still complain the net is slow.
I would say that the output drops on the port are already meaningful because:
each output drop frame is simply lost, the IP packet carried inside the ethernet frame can be part of a TCP conversation.
So a dropped packet is detected at the TCP level and this requires retransmission.
when the probability of drop is not so small all user TCP sessions are affected and users complain of resulting slowness = low TCP goodput
This is the kind of reasoning you should offer to the management, if users complain of slowness it is a problem this is the point regardless that there is no red alarm on the module or other direct indicator.
I've found a useful link that can help you in explaining the issue when servers are involved (it should be in your case if I remember correctly)
it provides a show command
show counters interface gigabitEthernet | include qos3Outlost
and a workaround to reduce the effects of oversubscription (to be tested so I would say that it can be not effective I wouldn't mention this to management)
Hope to help