If you look at the plug with the ground pin (the large round one) facing downward, the flat pin above and to the right is the common wire (not the same as ground) and the one on the left is the hot wire.
In a plug without a ground pin, modern electrical code states that a 2 pin plug will be polarized. If you look at a modern 3 pin wall outlet, the circular hole is for a ground pin. The left flat pin (common wire) is larger than the right flat pin (hot wire). A two pin plug is supposed to go in only one way.