In the Commonwealth Period, coinage was real money with a value equal to the face value in terms of gold or silver content. Coins were made to a strict controlled weight before circulation. With use weight was lost with handling.
Counterfeiters rarely achieved correct weights, so modern collectors usually check this point first. If a weight is within a range of 0.4 grams below target weight or above target weight it could be genuine. Then one should check the coin for certain characteristics which should be there for any given date. For example coins 1655 or 1656 usually have large 6’s in the date. View the section on distinctive markings for more information.