One way to tell is to look for a small 1 printed on the board next to the connector. The other popular way is to look for two notches in the cable, and plug the cable in so that the notches are on the same side where the notch of the connector on the board is. On some drives or motherboards, it's difficult or impossible to tell. Just plug it in. Then, connect a power connector to the floppy drive.
Most floppy drives use a slightly smaller power connector than the other drives.
Your power supply should have one connector that looks different than the rest. Connect it to the floppy drive's 4 power supply pins, flat side down. Turn on the computer. This should fix it. If not, the cable into the motherboard could be reversed.