Check if a binary tree is a binary search tree or not
Some quick notes:


Respect "Egyptian Braces" for your block structures (if, while, ...) i.e., the opening brace should be on the same line as the signature statement. The style you used (opening brace on a new line) is ordinarily used for C#, not Java. 
Node could and should be as immutable as possible:

class Node {
    final int data;
    Node left;
    Node right;
    Node (int x) {
        data = x;
    }
}

isBST is not a good class name. Class names are usually nouns. I'd suggest (as was already in the currently deleted other answer) BinarySearchTreeUtil.
getMin and getMax are factually only correct if you assume that the Tree is actually a BST. Only checking for those two is not a sufficient indicator, though. A BST is only fully a BST if each and every node satisfies the condition you propose. Your code does not check for that.

You should instead verify that each subtree is a BST recursively. Aside from that you're also assuming that the BST is ordered with the smaller elements to the left. It's also correct for a BST to be ordered with smaller elements to the right. Your code doesn't deal with that either.


Overall your code does not correctly solve the problem. To remedy this I strongly suggest you invest some time into unit-testing your code. Furthermore, you're disregarding conventions about bracing (but you're consistent so it's somewhat acceptable), as well as class-names (that's not acceptable).