you may check
steal.dev !== undefined
But that is not the solution. the right thing would be include different files while developting and buiding. It seems to me that there were feature of steal to not include some files in build, so it shoud be possible to achive needed result with this.
For now I use build task (grunt.js task that runs after steal build) that replaces stuff in production file. It is interesting how others cope with such problems.