You will know. You will have experienced derogatory comments by co-workers or supervisors about your age. These comments might be about your ability to do the job because of your age, questions about when you plan to retire, and general negative statements about old people. But these comments aren’t enough. You also need to have suffered some adverse employment action motivated by this discrimination. You need to have a clear sense that the negative action (demotion, firing, loss of pay or benefits) was taken against you specifically because of the age discrimination and not some other reason, such as poor performance, company wide layoffs or reductions in force, etc. If you feel that you are being discriminated against because of your age, you should contact an attorney to discuss the specific facts of your case. These situations are all time and fact specific and you should get legal guidance before you proceed.