The spell check works as you type in the posting window!
Anything wrong will be highlighted with a red underline squiggle. Checker is based on the location setting of your device e.g. because I am in UK it will check for double letters like dialed and advise me to change it to dialled.
To make the corrections right click on the red lined text and a dialogue with the suggested correction will appear. So its not a case of placing your cursor at the beginning of the text and hitting a 'check' button like it used to be on old word processing programs and such like.
The same principle is applied to mobile devices too, except they will automatically pop up the corrections suggestions as you type.