Décision tree, How to see under/over fitting with just looking at the leafs?
You can look at number of leaves. If number of leaves are very high compare to number of classes available for classification then tree might be overfitting or vice versa. 

You can also check number/percentage of training examples correctly classify by each and every leaf. You can put some threshold on top of that so that every leaf has to classify threshold of examples correctly.

But again, cross validation and holdout method will give you better idea for overfitting or underfitting.