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Ugh... PDF is the worst possible format to convert FROM. It was designed as an output format. This subject has been beaten to death around here because a lot of PDFs aren't tagged PDFs - meaning that letters (and a lot of times small groups of letters) resemble something like floating objects on a blank paper, each with their own coordinates and extra baggage. So it's very difficult to get a 1:1 conversion. A lot of formatting will be lost, some will get interpreted wrong, etc... Doing this in batches and not taking the time to do a proper check is a bad idea. Why do you need them as ePub anyway? Knowing that you could ruin formatting.
The closest thing to what I think you're looking for is Adobe Acrobat's "Save As - Optimized PDF - Audit space usage". An information window will pop up and if it says there that images take up some crazy amount like 98-100%, chances are that the PDF is "image-based". But then again, if the book is chuck full of pictures, the filesize is usually a good indicator too... And you don't need $200 for that, you could simply right click the PDF file and choose Properties.
Also, in any PDF viewer you could press Ctrl+A to select everything and just scroll down a few pages. I'd say if the text in the first 10 pages or so is highlighted in blue (or whatever theme you have set), it's "text-based".
If some pages are images, some are text, then it's a sh*tty PDF.
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