PowerPoint 2010 has a feature which check for spelling as you type in your slides. If you want to enable this feature then you can enable using the PowerPoint options.
Click the File menu –> Options link.
In the PowerPoint Options window, click the Proofing tab and navigate to When correcting spelling in PowerPoint section.
Mark the checkbox labelled as Check spelling as you type. This would enable the feature and PowerPoint would check for spelling errors as we type in a slide.