The best way would be to find and query the AcroRd32.exe executable. The file version information is stored in the executable itself.
There are lots of places in the registry that you could also potentially access relevant information.
For example the uninstall string:
Contains a string that you could use to search for the application version.
How you would do this in .NET I'm not sure but you would use some kind of text search command to search for the relevant text and then use "if exists" condition.