Making an accurate diagnosis of occupational asthma (OA) is, generally, important. The condition has not only significant health consequences for affected workers, but also substantial socio-economic impacts for workers, their employers and wider society. Missing a diagnosis of OA may lead to continued exposure to a causative agent and progressive worsening of disease; conversely, diagnosing OA when it is not present may lead to inappropriate removal from exposure and unnecessary financial and social consequences. While the most accurate investigation is specific inhalation challenge in an experienced centre, this is a scarce resource, and in many cases, reliance is on other tests. This review provides a technical dossier of the diagnostic value of the available methods which include an appropriate clinical history, the use of specific immunology and measurement of inflammatory markers, and various methods of relating functional changes in airway calibre to periods at work. It is recommended that these approaches are used iteratively and in judicious combination, in cognizance of the individual patient's circumstances and requirements. Based on available evidence, a working diagnostic algorithm is proposed that can be adapted to the suspected agent, purpose of diagnosis and available resources. For better or worse, many of the techniques - and their interpretation - are available only in specialized centres and where there is room for doubt, referral to such a centre is probably wise. Accordingly, the implementation or development of such specialized centres with appropriate equipment and expertise should greatly improve the diagnostic evaluation of work-related asthma.