In the sentence "The tennis pro showed us how to return a serve.", which word is the indirect object in the sentence?
'us' would be the indirect object in this case.
You can tell since, though it may not sound as natural, you could place 'to' before 'us'- 'The tennis pro showed to us how to return a serve'.
In some other languages, e.g German or Latin, the indirect object is used with the dative case, and the verbs used with these are verbs including 'to give' or 'to show'.