You can easily tell if your water level is too low if air is being sucked through the skimmers.
Another tell-tale sign is if you can see air or bubbles coming out of the jets.
The correct water level is in the middle of the top tile, near the coping of the deck.
If the water level is too low and the skimmers are sucking air, turn off the pumps immediately and add water to the pool. It does not take very much time at all to ruin the equipment when air is in the system, so be careful to keep an eye on your water level!
Why This Is Important: This is important to keep optimal levels of water to the pumps and thus, avoid burning it up.