I know of no way short of opening the projects and inspecting the plug-ins. There is a VST drop down in the standard header of a VST plug-in. In SONAR 6 it looked like this (the VST drop down is below the "v1.cmp" in the title bar to the right of the preset controls)

DX plug-ins do not have the drop down.
If you do not have access to the old DAW and
- you are running 64bit SONAR (or 32bit SONAR and did not install the DX format plug-in) and
- Kontakt 2 can be loaded into a new project and
- old projects throw a missing Kontakt error messages
it is reasonable to assume the old projects are looking for the DX format plug-in.