STL files contain no scale information. It's just a dimensionless shape. It's up to you to interpret what its units are supposed to be and scale it according to your expectations and needs. A 100x100 surface might be 100x100 mm, or it might be 100x100 km. Depends if it's a topological scan of a machined part or geographical data.
That said, there are sometimes nonstandard things crammed into STL files, so never say never. Some files may have units specified in the header text, but there is no standard regarding if or how they will be presented. That said, the built-in decoder does not return the header text, so you'd have to use something else if you want to find that information (if it exists). Automating its retrieval would take some foreknowledge of how it would be presented. I doubt any of the PDE toolbox tools attempt to look for such inconsistent, nonstandard information.
