What worked for me best was to have as thorough monitoring as possible (IO, mem, CPU, per process, etc.) on infrastructure side and as complete event sourcing o

What worked for me best was to have as thorough monitoring as possible (IO, mem, CPU, per process, etc.) on infrastructure side and as complete event sourcing on the app domain side. It's easier said than done of course, and in a brownfield situation you will never achieve perfection in terms of coverage. Once that's in place, write tests with selenium or unit test framework with parallel extensions (depending if cases are frontend or backend based) and analyze using home made tools.