Looking up your local Administrator name:

wmic path win32_group where (LocalAccount=true AND SID="S-1-5-32-544") get Name

edit: that's just the name of the local Administrators group, here's how to get the Admin-accountname:

wmic useraccount where (domain='localhostname' and sid like 'S-1-5-21-%%-500') get sid /value

wmic useraccount where (domain='localhostname' and sid='sidfromfirstcommandhere') get name /value