What Makes a Senior Software Developer?
The common thing i can see about seniors developers are the length of his code.<p>A junior developer doesnt divide too much his code, repeat himself, the avg length of his code is high, etc.<p>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.<p>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.<p>Of course this seems a bit qualitative too, but its like a pattern i noticed.<p>Anyway, the preferred method here is: Developers categorize others. Between a team, or a company.