Assume any object with pure black diffuse, and PBR=true is metal

[FireFoxxG]

Sphere and mirror text environment

http://demos.janusvr.com/firefox/JanusWebTesting/PBRtest/ProbeTester.html

Metals ALWAYS have pure black diffuse. Ideally anything black would operate like a metal(modulated by the color and brightness of the specular). A decent stopgap is to assume anything with a pure black value(not texture) specified color is a metal. 95% of the time you dont really use a diffuse texture with pure metal.