how do you come to know wether chemical reaction has taken place or not?

We can tell if a chemical reaction has taken place when one or more of the following things happen:

• There has been a colour change inside the reaction flask.

• A gas has formed. Usually we know a gas has formed when we can see bubbles. This should not be confused with boiling, which only happens when a liquid is heated to its boiling point.
• A solid has formed. Usually we know that some solid material has formed when we can see a sludgy or cloudy deposit, or crystals forming.

All the signs listed above are visual, or recorded by sight. That means we can see them. Our other senses can also help us to say whether or not there was a chemical reaction:

• Sometimes chemical changes can be smelled, for instance when a new material, that has a strong smell, is formed.
• Other chemical changes can be felt, e.g when the reaction produces heat.
• Some chemical changes can be heard, e.g. when an explosion takes place.