Rely on those (smart people) that have gone before (and use their research)... Meaning using references and the corresponding dates in places like RIC, etc.
The dates on Roman coins can be narrowed down by using the legends on the coins. For example you might see COS VI on a coin. In many cases you can find out on what date the emperor was made consul for the sixth time and for the seventh time. This can give you a date range for the minting of your coin. This is a range only and precise dating is not always possible. For example it might have taken a while for the mints to get the message from the emperor, the celators to engrave the new legends, and for the coins to circulate. Here is an excellent book that can help answer your question. It includes 2 books by Zander Klawans. https://www.amazon.com/Handbook-Anc...-2&keywords=handbook+of+greek+and+roman+coins
Many scholars over a couple of centuries have worked on that for us. If you can identify a coin type, scholars have assigned it as narrow a date range as they can and you can just look it up. There is even a massive book, Dated Coins of Antiquity, by Edward E. Cohen, about the ancient (mostly Greek) coins that actually have dates on them. Of course, the dates do not use the modern era, so that book explains how the dates on those coins correspond to modern reckoning. Be aware that it focusses almost entirely on Greek coins, not Roman imperial coins. Also, most ancient coins do not have dates explicitly on them, and that book does not cover them at all.
Some ancient Roman coins can be dated to within a very short time. Sometimes even within weeks by looking at all the titles attributed to the person on the coin and looking for overlaps. You might have a COS V and IMP and whatever other titles you see on the coin. They are all well documented. It's all part of the fun of the research involved in ancient and medieval coins.
