Nothing. Honestly. Spellcheck can be very useful for skimming through and catching obvious spelling mistakes, but it’s not as good as a human being. Just like most technology, its specific uses are matched by its faults. And as long as you recognise its limitations, that’s fine.
Spellcheck will say that ‘form’ has been spelled correctly, but not that it should have been ‘from’. Spellcheck does not put words into context.
When proofreading, context and sense are very important; correct spelling does not achieve either.
Spellcheck will pick up many spelling errors and can save you time, although it should be used as an aid rather than a replacement for proofreading.