Depends on what you're using, older versions of MS Word didn't do this, the latest version (2016) will check as you type and pick up most errors, very similar to predictive text. You can also add your most common mispellings to the checker so it automatically puts it right. I only use Word and a dedicated word processor in our bespoke software at work so don't have much experience of other current wp's available.
My system, Ubuntu, underlines in red any word it doesn't like and a right mouse click on it reveals a list of word from which to choose. Word checkers that automatically substitute what they think is correct often fail by using a correctly spelt but wrong word.
Sorry, should have bee more precise!
On the forum . The Spell Check button just appears to check what has already been typed rather than turn on "check as you type"
Just remember that correct spelling is only part of writing correctly, using the right words and proper punctuation is also important.
Always proof read before hitting 'submit'.