It goes without saying that, if you're teaching ESP (English for Specific Purposes), you'll see the need to understand your students' profession, especially the job skills that involve oral and written communication.
And that is where this wondrous website comes in:
From what I gather, the Canadian government listed the essential skills for a myriad of careers. Granted, this resource won't be all you need to fully apprehend your students' needs and aims, but it seems to be a great springboard for a needs analysis.
For instance, if you are teaching hairdressers, you'll see there that their main focus regarding language should be oral communication, so they can talk to their clients. However, there's also a need for reading manuals and labels. And the website doesn't stop there. It'll even list you the subskills and some of the typical tasks!