Can't explain it as good as Narue does here, so you should take an hour or two to read the article. Everything will be clear afterwards

If you understand the concepts and have written the insert function, then this should not be all that difficult to understand. Just for reference:
So one solution would be to write a function that takes a node, at any given point in the tree, and can calculate its height (might involve recursion depending on how you determine it). Then you can pass child of the root to this function and determine if its a proper AVL tree or not.

Below is some scrappy pseudocode:
if heightLeftSubTree - heightRightSubTree is between -1 and 1
--this tree is an AVL tree
else
--this tree is NOT an AVL tree
end

Hope that helps (without giving to much away )