"How can one tell if a core module is purely core or dual-life?"

Take a look at a post of mine from last week which discusses this. It has other information, and does not use the term "dual-life". See the two sections which start with:

• "Modules which are core, but do not have a separate CPAN version, ..." [i.e. purely core]
• "Modules which are core, but also have a separate CPAN version, ..." [i.e. dual-life]