The Spell Check tool uses statistical analysis to determine if words are likely to be misspelled. For example, if you have a word that only differs from another word by one character and one word is used a lot and one is only used once, it can be fairly certain that the used-once word is incorrect. Note that is has several “rules” for determining if a given word might be incorrect - You can see the rules by going to the WordList and hovering over the Tools > Spell Check menu item.
As you mark words correct/incorrect in the WordList or add more text to the project so that it has more information, it should get better at determining which words are misspelled.