In the technical preview Microsoft released to the public, the spell check does not work. To fix this, delete the Overwrite key located under:
HKEY_CURRENT_USER\Software\Microsoft\Shared Tools\Proofing Tools\1.0\Override
Close all office applications then reopen one like word. Navigate to options:
In Options click proofing then click recheck document.
From here on out provided you have check spelling while you type selected it will automatically underline spelling mistakes in red.