I think the question is really about how to structure the WMI query so that it returns true when no results are returned. (I would add "WQL" or "WMI" or both as tags to the question).
One of the best ways to get some experience with WMI querying is to download the WMI Code Creator from Microsoft. Of course you have to run this on a windows box, but you'll be able to zero in on the query you need to feed into the Nagios plugin using the GUI.
The querying language used for WMI is WMI Query Language (WQL), similar to SQL you can query whether a particular eventcode exists within the last 48 hours. Here are some useful links about syntax that is acceptable for WQL.
WQL Keywords: link
WQL Operators: link
WQL Supported Date Formats: link
Event Class: [depends on what you're looking for specifically]
TargetClass: Win32_NTLogEvent link
You'll be using the most common root\CIMV2 namespace, and the class you'll need is the Win32_NTLogEvent class to obtain the information you're looking for. The rest is just the structure of the query.
Since we don't know which particular event you're looking for there are a couple of properties you can use to change up the query.
Logfile = Which Event log do you want to look in? "Application" or "System" etc...
User = Who generated the event? "NT AUTHORITY\SYSTEM" or maybe you're looking for someone specifically.
You can narrow the query using the WHERE clause, just like in SQL, using the TimeGenerated property. TimeGenerated is in IntervalFormat (or UTC format) link.
Here is a quick guide on working with Dates and Times using WMI. link
WHERE DateDiff("hh",TimeGenerated,GetDate()) < 48
So to put all that together it should look something like this.
SELECT * FROM Win32_NTLogEvent WHERE EventCode=4001
AND DateDiff(hh,TimeGenerated,GetDate()) < 48
4001 is just a made-up number, look up the event ID for what you're wanting to query on. ;)
You can add additional AND statements to include properties to narrow the results as needed. This in addition to Phil's answer should get you where you need to be.