You have to look at the number of each element on each side and see if they are equal.
For example in the following equation:
N2 + H2 = NH3
If you compare the Nitrogen’s, you see that on the left side you have 2 and on the right you only have one, so you would need to balance this equation.
An example of when you don’t need to balance:
K + H2 -> KH2
In this example, you can see that there is only one potassium (k) on each side. Now, you compare the hydrogens. We can see that there are two hydrogens on both sides. Since both sides have the same of each element, we do not need to balance this equation.
IN SHORT, balance if there are different numbers of one or more elements on both sides. Don’t balance if there are the same amount of each element on both sides.