How do you tell if you are overdosing on vitamins?

May be hard to tell. In general, vitamins are tolerated at pretty high doses. However, large doses can cause many problems, from interfering with medication effects to a variety of general symptoms that are hard to separate from other illnesses (e.g., stomach aches, numbness and tingling, and others). It is rare that you would need more than 100% of the recommended amount of any vitamin, but check it out if unsure.