Provable how? Not raising developer salaries might be indicative of there being no shortage of developers, but it's not proof by any stretch of the imagination.

Provable how? Not raising developer salaries might be indicative of there being no shortage of developers, but it's not proof by any stretch of the imagination. In markets of asynchronous information, supply and demand fails. It's quite possible for developer salaries to remain constant or even decrease at the same time as demand for good developers goes through the roof.