AMH (Anti-Müllerian Hormone) is a hormone secreted in the granulosa cells (satellite cells which surround eggs) and its robustness and stability throughout the cycle makes it the most important parameter in terms of predicting ovarian reserve.

A simple blood test taken on any day of the cycle will give you this important information (best is at day 3 because you can measure several more hormones at this point).

Your AMH level will also help determine 1) how well you will respond to a hormonal stimulation (a high AMH will generally respond well to follicle stimulating medications), as well as 2) tell you how many antral follicles you can expect to see on your ovaries and how active they generally are. More you can find here: Is it possible to increase AMH levels?