History may be enoug. To diagnose IBS there may be enough historical data as well as physical exam data that will guide physician to this diagnosis. Without tests, there is a potential that some other diagnosis may be missed, so if the treatment prescribed is not relieving your symptoms or you have other ones popping, return back to the office.
Debateable. There are criteria that suggest ibs, but their use is debateable. Some drs feel that IBS should only be diagnosed when everything else is ruled-out, a diagnosis of exclusion. Others feel that if certain criteria are met, it is probable. Most doctors will run some tests first - liver, thyroid, etc, but in young, otherwise healthy person may choose to try common treatment before invasive tests.