How do I find whether or not spot colors have been used in a PDF?
With Adobe Acrobat choose
View > Tools > Print Production and click Ink Manager
I don't believe apps such as Reader or Preview will allow you to verify colors.
Photoshop rasterizes everything in a PDF when you open it unless it is a Photoshop PDF saved with editing capabilities enabled. There's no way to use Photoshop. In fact, often Photoshop will degrade a PDF if used. Using Photoshop to verify colors within a PDF is haphazard at best.