Alert to notify if SNMP is not responding?

I have not been able to find a direct way to do that, however if you change the Status and Response Time polling method from ICMP to SNMP, the node will report down if SNMP polling stops working for that node. If you choose this method you may wish to change the text of any related alerts accordingly. You can change this on a node by node basis under the node's List Resources page, or globally under the Manage Pollers page under Settings.

There are a few alerts that people created for this as well as some other checks .. here is one I found to get you started

Re: NPM does not alert when a Windows device goes unresponsive when polling method is SNMP

Alert on Nodes that stopped responding to SNMP here is a nice thread with some solutions and tweaks .. everyone kinda does it a little different

Alert me when a managed node last poll time is 10 minutes old

Thank you I believe this is exactly what I was looking for.