How can I check for a pending reboot?

Pending reboot can be caused by variety of reasons, not just the ones that are detailed in other answers. Try PendingReboot module, which incorporates various tests into a single cmdlet:

# InstallInstall-Module -Name PendingReboot# RunTest-PendingReboot -Detailed

You need to check 2 paths, one key and you need to query the configuration manager via WMI in order to check all possible locations.

#Adapted from https://gist.github.com/altrive/5329377#Based on <http://gallery.technet.microsoft.com/scriptcenter/Get-PendingReboot-Query-bdb79542>function Test-PendingReboot {    if (Get-ChildItem "HKLM:\Software\Microsoft\Windows\CurrentVersion\Component Based Servicing\RebootPending" -EA Ignore) { return $true }    if (Get-Item "HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate\Auto Update\RebootRequired" -EA Ignore) { return $true }    if (Get-ItemProperty "HKLM:\SYSTEM\CurrentControlSet\Control\Session Manager" -Name PendingFileRenameOperations -EA Ignore) { return $true }    try {         $util = [wmiclass]"\\.\root\ccm\clientsdk:CCM_ClientUtilities"        $status = $util.DetermineIfRebootPending()        if (($status -ne $null) -and $status.RebootPending) {            return $true        }    }    catch { }    return $false}Test-PendingReboot

Your syntax wasn't correct, if you want to run the PowerShell command from cmd, it has to look like this:

powershell.exe "Get-Item 'HKLM:SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate\Auto Update\RebootRequired'"

But like Mathis mentioned, this key only exists if a reboot is pending.