While the rules to determine if protons are homotopic, enantiotopic or diastereotpic are quite simple they may be difficult to apply.

In this webpage you can draw a chemical molecule and find out where the diastereotopic atoms are. Go over a hydrogen with the mouse to find out what the NMR equivalent atoms are.

You can also directly paste a molfile or a SMILES by going over the molecule drawing tool and paste using the keyboard shortcut (CTRL + V).