The difference in head size is quite dramatic, and you get a feel for it. However, 3 quick ways of spotting it:
1. the point where the chins meet the neck is well over halfway down on the 2d, about halfway on the penny
2. the back of the head is vertically over the back edge of the lower bust on the 2d, but WELL to the right of it on the penny
3. On the 2d, the chin is vertically above the front of the lower bust, on the penny it's the tip of the nose
It does make you wonder why the same bust wasn't used on both denominations.