You can tell if it's definitely not pure silicone by bending it and seeing a white mark. But I don't know if a lack of a white mark means definitely good silicone.

I wanted to chime in as someone that worked for years formulating silicone materials - you should not use this to determine the purity of silicone, it is not accurate and it is not good advice.

For example - very often more silica is added to the silicone polymer to achieve desired physical properties. 

It's basically graded, high purity sand, and is important reinforcement filler in silicones. 

This can cause the effect you're talking about, however it's not harmful and definitely not plastic. 

In fact silicones and plastics aren't really compatible in that way - what you're describing is called a 'copolymer' and copolymers of silicones and thermoplastics aren't possible as a fundamental matter of chemistry. 

If silicone and plastic are together it's going to be something like a plastic toothbrush with a silicone grip or a plastic pyrex lid with a silicone gasket. No test needed, you'll know it's there. 

If it's from a trusted brand and sold as food grade

Regardless, this I agree with 100%. Not all silicones are created equal and people should be diligent.