Here is a post which describes how to monitor HSRP state using PRTG network monitor.
Add SNMP custom (see this post for screenshots: http://www.madari.co.il/2014/04/prtg-monitors-pps-on-cisco-routers.html) with the following SNMP OID:
The sensor will retrieve integer value which will represent the state of the HSRP group.
And the result:
As we can see the router returns the value “6” which means that he is in active state, a value of “5” will indicate that he is in standby state.
Here is the SNMP object information taken from Cisco SNMP Object Navigator:
MIB CISCO-HSRP-MIB ; - View Supporting Images this link will generate a new window
Description The current HSRP state of this group on this interface.