How can I check for a pending reboot?

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.

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:

# Install
Install-Module -Name PendingReboot

# Run
Test-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