The Fleep native apps for Windows and Mac have spell-checking built into them. All you need to do is make sure they are enabled.
Enable spell-checking on the Mac app
Click on the Edit menu item, hover over Spelling and Grammar and change the settings according to your preferences:
Enable spell-checking on the Windows app
Click on the Settings menu item, and in the popup that appears check the box for Check spelling. As noted, the setting will be applied after you close and restart the app.
Note that if you are using Fleep on the web browser, then you'll have to use the browser's spell checker to check your spelling.