There are many ways to determine if a PDF file originated from a scanned page:
The Page Appears to be Skewed
Sometimes sheets are not properly fed into the scanner. The result is the page appears to be crooked, or skewed on the screen . Lines of text will not be straight but will appear to slant up or down.
Search for Characters that Appear on the Page
Use the find command in Acrobat to search for text that appears on the page. Select Edit > Find and type a term that appears on the page in the search field.
If the document was scanned, Acrobat will not find the search item but will display the message: “Acrobat has finished searching the document. No matches were found.”
Zoom in and Check for Jagged Edges on Smooth Characters
Scanned images are bitmaps. The edges of curves on bitmapped images will not appear to be smooth or rounded but will be jagged, as shown in the sample illustrating the word “Writing” in the screenshot below.
Use the Marquee Zoom tool in Acrobat to define the area and magnify the edges of curved letters such as “c”, “s”, and “o”.
Text that has undergone the OCR process using the ClearScan option will display edges that are smoother but still uneven or lumpy where there should be smooth curves, as shown in the illustration of the of the words “Quality” and “region” in the screenshot below.