Question

How can we know that'Here we have to use equations[V=u+at, etc] and here we have to use laws of newton

Answer 1

v= u+at is one of the kinematical equations which we have to use to find velocity or initial velocity or acceleration or time . While laws of newton are to be applied according to questions asked.

Answer 2

There is a simple way to find out. If the questions ask for finding initial velocity or final velocity or acceleration or time period or displacement or distance, then you apply the equations, these equations are commonly known as SUVAT equations. 
(Remeber there's a catch, acceleration should be uniform)