I think that there will be some definite observable properties that will be evident when heat energy is added to water. Some of the first changes will be with the water, itself. There might be a bubbling or a movement of water. This is one of the first steps regarding a phase change. In this case, the addition of heat energy is moving the water from a liquid phase to a gaseous stage of matter. The percolation of water will start off slow and then increase in intensity. This will also result in the substance vaporizing on the side of the container. You might see some vapor collecting on the side, making it difficult to see. As more heat is added, the water will continue to bubble and steam will result. This is where the most definite sign of a phase change is evident, as the form of water is changing from a liquid to a gas, or vapor. The water molecules are being "liberated" into the air. One can still that water is there, but in a different form. If you put your hand over the water, the vapor that collects on your hand is liquid, demonstrating that water is still there. It has not changed, merely moved from one form, or phase, to another. After a prolonged period of time, all of the water will evaporate into the air as a gas.