I have tried to put together some pointers regarding a guery w.r.t  Scalability Testing. Its not comprehensive but it may give you a few good leads to start with.

Scalability testing is an extension of performance testing. It is done to identify the major bottlenecks with increasing loads. Performance testing is first conducted as a baseline for this and then when a scalability testing is conducted the relative performance degradation can be monitored.
A scalable application has a response time that increases linearly with load. For such applications only hardware upgradation will be necessary to handle increased loads. If the linear relation is not there, then there exists a bottleneck that needs to be identified.
Typically scalability testing is done by first measuring the optimal response times by doing performance testing. The application is then loaded with a scaled up load and by increasing the hardware capacity in terms of processor speeds/numbers, memory, bus speed etc. If these changes are sufficient to keep the performance optimal then the application can be certified as scalable.