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 and also with people filing the edge of the coin to take some metal for themselves before passing the coin on at full value. Very rarely will you see a coin cut in half. You may see a straight edge in places which may suggest the coin has been “clipped”.
Counterfeiters rarely achieved correct weights, so modern collectors usually check this point first. If a weight is within 6% of the target weights above 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