It looks like there are a number of plugins at Nagios Exchange to check for available updates--for example, this one. I've used this one in the past, I believe. That's not precisely what you're asking, though; those are whether updates exist, and you're asking whether or not the system is checking for updates.
What might be closer to what you're looking for is something like this PowerShell script:
$lastcheck = Get-ItemProperty -Path Registry::"HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate\Auto Update\Results\Detect"
$lastdate=[datetime]$lastcheck.LastSuccessTime
$currdate = (Get-Date)
$status = new-timespan -start $lastdate -end $currdate
If ($status.days -eq 0 -and $lastcheck.LastError -eq 0) {
Write-Host "OK: Wuauserv last ran $lastdate."
$exitcode = $status.Days
}
elseif ($status.days -eq 1 -and $lastcheck.LastError -eq 0) {
Write-Host "WARNING: Wuauserv last ran $lastdate."
$exitcode = $status.Days
} else {
Write-Host "CRITICAL: Wuauserv last ran $lastdate."
$exitcode = 2
}
exit $exitcode
(You'd want to check that the date was within the last 24 hours (the default interval is 22 hours) and that the last error was 0.)