Synthetic attribute in .class files

I've been parsing debug info in .class files in order to generate a
mapping between Scala source positions and emitted classes. Synthetic
classes, e.g. those generated for case classes, have bizarre debug line
numbering that throws off my mapping.

Given a .class file generated by the Scala compiler, is there a reliable
way to check if it is based on a synthetic class?

Thanks,
Aemon