How do you know when a substance is boiling?

When you are boiling eggs for example the pot with water is hot and you can see bubbles in the pot. This is called boiling water. When the eggs are in there they are boiling to make hard boiled eggs. Depending on how long it is in there determines the outcome. On the surface of the egg shell you may also notice little bubbles all around the shell which means it is boiling.

Hope this helps! :3