Dative or ablative - how to tell?

[vivod]

I'm sure there is a simple trick I'm missing. Using the apt phrase 'docendo discitur' as an example, how does this translate to 'it is learned by teaching' and not any other of the uses of the dative or ablative, such as 'for' or 'with'?

It's easier with a preposition but when one is not present, is the correct method to find the most apt definition or something else?

 

[Imperfacundus]

By context. E.g. in the example given, none of the interpretations involving the dative make sense.

 

[vivod]

Super, thanks