Size is the easiest way to tell, but there are subtle differences in the proportions too. Look at the 3 lions in one quarter of the arms and how much they fill the area for example. The sixpence was only made in this style during 1887 and was withdrawn almost immediately - not because of the problems of telling it from a shilling but because the criminals were gold dipping it and passing it off as a half sovereign!
If you are American, remember that the sixpence is about the size of a nickel, the shilling a quarter, florin a half dollar and half crown a dollar. Threepence is smaller than a dime. Then the farthing is a nickel, half penny a half dollar and penny a dollar size.