Your question's pretty broad. There's a lot of different commands you can use to troubleshoot and monitor QoS, so I'll focus on the primary question you have, which is how to reasonably verify your QoS configuration is working and how to read the policy-map interface output.
The only true way to verify that QoS is working is to hook up a traffic generator and monitor your drop rate in various queues. Since that isn't typically feasible, particularly in a production environment, all you can really do is verify that the traffic is being marked and classified properly.
What you're really looking for, when it comes to verifying if your QoS configuration is working, is for the counters in the policy-map interface command to increment.
So, for example, in the output your provided:
Class-map: VOICE (match-any)
3860628 packets, 1070196895 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: protocol sip
97348 packets, 49867304 bytes
5 minute rate 0 bps
Match: protocol rtp
3763280 packets, 1020329591 bytes
5 minute rate 0 bps
Match: access-group name NEC-PBX
0 packets, 0 bytes
5 minute rate 0 bps
Priority: 40% (340 kbps), burst bytes 8500, b/w exceed drops: 5
You can see that you're seeing packets under SIP and RTP, but not NEC-PBX. If you know you're getting SIP and RTP traffic across a link, you should see the packet counts increment and that's a reasonable way to know that your configuration is basically working.
Best Answer
If you're just looking for interface output queue drops, you can use the cportQosDropPkts OID (part of CISCO-PORT-QOS-MIB).
If you have a CCO you can look up the OID details here.
This OID shows a depricated, but it should work on the platforms that you've listed in your comment to @Raul D.
Additional required MIB files:
CISCO-QOS-PIB-MIB.txt
CISCO-SMI.txt
CISCO-TC.txt
CISCO-VTP-MIB.txt
Example using snmpwalk and bash on a C3560: