Files formatted as a presentation (e.g., ppt and pptx) can be found by advanced searches in Google (searching for the article title in quotes, followed by the keyword filetype:ppt), or by searching in the internal search engines of sites like SlideShare, a website for sharing presentations, or other similar websites. See: Powerpoint presentations
Scientific presentations may be found in this way even if they are not published in conference proceedings. They may include educational presentations, which can be valuable for tracking the popularisation of research outside the formal publications.
The main drawback of presentations is that many scholarly presentations are also available in generic formats, such as PDF or HTML, and many others. It may not be practical to identify presentations in such formats.
When uploaded to Zenodo or Figshare, you can use their usage indicators.