If 4 points r coplanar we have many technic to find but the shortcut is to lock at the points
Example : Let 4 points A(1,1) , B(-1,1) ,C(-1,-1), D(1,-1)
Here, A is in 1st co-ordinate , B is in 2nd co-ordinate , C is in 3rd co-ordinate and D is in 4th so if the 4 points had same - or + number for x and y as in P(x,y) then they r coplanar