Local Maxima and minima doubt pls help!!!!!!!!!!!?

Okay to find local maxima and minima of the function,I first differentiate the equation and i equate it with 0 and then I find x.I then again differentiate the function and then by second derivative test I find the maxima and minima.But in many books I that I referred,it was given that we can find the minima and maxima just by first derivative test.How to check it only by first derivative test??Also is it ok if I always double differentiate and find the maxima and minima??