Determine whether the extremals of a functional are weak or strong?

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Is there any thumb rule for determining whether the extremals of a functionals (which are determined using Euler Lagrange equations) are maximum or minimum, weak or strong.
I read about Weierstrass function and Legendre condition in Gelfand and Fomin's Calculus of Variations but didn't understand them.
Is there any shortcut to find out whether the extremals are maximum or minimum, weak or strong.
Or any equivalent condition for same. Please Help !