Just like with Word, PowerPoint has a built in accessibility checker.  This link to Accessibility at the Microsoft website can show you how to run it for Macs or PCs.  It also lists some of the most common issues and fixes for PowerPoint.  Typically, those are alternative text and reading order.

Alternative text is what is read by a screen reader to describe an image in a presentation.  If there is no alternative text a screen reader will read there is an image and the size of it.

Reading order is the order in which a screen reader reads the information on a page.  And while it may seem intuitive to read from top to bottom and left to right, PowerPoint reads in the order which the information was added to the page originally.