Run the following command:

wuauclt /detectnow

To Report back to WSUS run the following:

wuauclt /reportnow

Reference: https://social.technet.microsoft.com/Forums/windowsserver/en-US/dfec25e8-c0fb-4909-a762-0c6661b50ce1/force-a-wsus-client-to-check-for-updates