A good general English dictionary with examples is probably the best way to differentiate words. (I don't mean a free online one). In some senses for example of 'respect' and 'regard', there are no differences at all in usage, while in other senses you would normally use only one or the other. When you know the meanings of each word, you can often tell how they differ.
By "a good dictionary" I mean one version up from the one you can afford.
Maybe someone else knows of a good synonym/antonym dictionary that is really useful.