The JVM can be instructed to emit diagnostics regarding JIT compilation. Dunno if similar things are possible with JavaScript engines. Apart from that, profiling. Coding style is being discussed to death in other comment threads here. In short, "boring code" (few closures and functional idioms, no reflection, careful use of dictionaries, no ultra-flexible function parameter lists, avoid regex unless absolutely required) avoids surprising (read: hard to optimize) behavior.

But maybe all of this is besides the point. It's unrealistic to do heavy duty processing in a dynamic language and expect that the JIT always saves us. In comparison, Python programmers suffer no illusions that their code is usually just "fast enough", and call out to external libraries when heavy duty processing is required. Modern browsers provide some of these optimized capabilities for JavaScript, and they should be used whenever possible.