Curses are tricky because, by definition, they are easy to confuse with natural happenstance. At the same time, it is easy to assume an external force is acting upon an individual when they have made a series of choices and are now dealing with the consequences. Also, there are a variety of severities, ranging from simple jinxes to the most lethal curses.
Diagnosis of being on the receiving end of a curse is straightforward: get a reading from one of your local metaphysical practitioners familiar with curses and their removal. Reading for yourself is possible using a variety of methods, but it comes with the risk of misinterpretation due to your proximity to the situation.