I am migrating from SNMP community1 to SNMP community2 on a NEXUS switch (plus a bunch of 2960s).
Is there a way to monitor that the old community is not used anymore?
The thing that you need to do is to erase the old community, i know that sound like you haven´t doit before, but if you do that, and only configure the new one, and on your management or monitoring software, place the new one to, you will be sure that only the community2 is in use.
You are using cisco, i guess you can debug your snmp proccess:
your_router#debug snmp packet
your_router#debug snmp request
With snmp request, i think, you can have the community name what you are looking for. With snmp packet, you will get the source ip.