What you'll be looking at is a report that uses SQL to generate the results.
On your primary polling engine, open the Database Manager (you'll find this within the SolarWinds application folder). Add in the default server (generally your Orion SQL server), and take a look at the contents of the 'SysLog' and 'SysLogNodes' tables (if you double click on the table in the left-hand pane of the Database manager, the program will automagically create a search select query for you).
The 'SysLogNodes' table will show you all the nodes that are sending SysLogs to SolarWinds, and the 'SysLog' table will show you the most recent log entry from these nodes.
If you construct a SQL query, which joins these two tables on 'NodeID', and show those where DatTime on the SysLog folder is older than say, one week (or whatever is your preference), you should see those nodes which are not sending SysLogs currently.
I've not tested this myself, as the instance I have access to today only appears to have one SysLogNode, but hopefully the above will help you get started