SwiftKey doesn't offer a dedicated spell check option within the app. However, SwiftKey should offer you correctly spelled predictions based on the word you are typing: for example, if you start typing 'neces' you should be offered predictions such as 'necessary', 'necessarily', or 'necessity'. In this way, SwiftKey does help to insert correctly spelled words.
If you would explicitly like to use spell check, then this is a feature you can generally enable in your device settings under 'Language & input'.
For more information on how to correct mistyped words, please see this article.