What Makes a Senior Software Developer?
The common thing i can see about seniors developers are the length of his code.A junior developer doesnt divide too much his code, repeat himself, the avg length of his code is high, etc.A semi-senior developer knows how to divide his code, have a good analysis and design, knows some common patterns, try to have good practices, and repeat himself a little. (Sometimes takes bad decisions trying to not repeat without sawing his design)
Normally have some methods with too much lines of code but not too much.Seniors developers are embrace testing a lot, even test his tests (mutation testing), are carefull about decisions who impact on architecture, and the average of the lenght of his code is between 5 and 8 lines of code (and always simply readable).
Cares a lot of dependencies and really knows how to use all of his tools. Specially when talking about refagtorings. 
Normally, they loved the job/books of some of these guys: Robert Martin (uncle Bob), Sandi Metz, Kent Beck, Martin Fowler, The Gang of Four, Alistair Cockburn and/or Alan Key.Of course this seems a bit qualitative too, but its like a pattern i noticed.Anyway, the preferred method here is: Developers categorize others. Between a team, or a company.