Heating water has its boiling point which will be at 100 degrees Celsius or 212 degrees Fahrenheit under normal and average conditions at sea level. Unknown to many, the boiling point of water and other liquids actually depends on the oxygen content and atmospheric pressure. This could mean how high or below you are from sea level. Higher altitudes require lower temperature at which water boils and this means probably less than 100 degrees Celsius or 212 degrees Fahrenheit!
Now this would be just for water. What about other liquids? Different liquids would have different boiling points and therefore would have different simmering temperatures. So differentiating simmering and boiling point with different liquids and mixtures would be difficult. The best way to determine boiling point will be through visual observation. Boiling, as most would know is the ‘violent’ bubbling activity of any liquid. Under this condition it is normally accepted as boiling.
The biggest question is when do you determine that the liquid is simmering? Again for water it is quite easy to observe. The water gently bubbles hardly emitting forceful steam even with whistling kettles. For other liquids, it depends on the ‘acceptability’ of gentle bubbles being observed. For thick liquids such as those made of tomato paste or sauce, frequency of very mild erupting bubbles should be the basis without the aid of a cooking thermometer.
Thick liquids such as those with mostly tomato sauce or paste will show up gentle bubbles for every two to three seconds. The liquid should be simmering at that state. Anything more frequent than that will ‘cook’ firm what is at the bottom. It is best then to have the liquid come up with gently erupting bubbles at three-second intervals to avoid ‘over-cooking’. This would also be the same consideration of ‘simmering’ thick soups or recipes that would have at least two-thirds solids, like meat and vegetables, in the pot.
To reiterate, liquids have different boiling points and different simmering points at the different levels from sea-levels and different atmospheric pressures. Unless you would have a graduated scale for different liquids and sauces with corresponding thermometer to monitor them, then it should be good. Other than that, keen observation of bubbles coming from the cooking process should help and be sufficient to determine the simmering.
A barometer could help in determining atmospheric pressure for some cooking techniques. Abrupt weather changes though could also change readings drastically in the process where food needs to be cooked for more than an hour or two. A good example for this would be cooking beans to soften up. It usually takes two hours to cook beans to attain the softness or suppleness desired.
Having your television or internet on, while cooking, might just help in being aware of the atmospheric conditions. It is still wise and best to train your eyes in observing when your cooking is at the simmering point.