Best method of changing content of one field based on whether another exists I have one field, field_foo, that I want to be rendered one way if field_bar does not have a value. Should field_bar exist, I would like to render the values of both field_foo and field_bar together inside the same markup that all my other fields have. 

I could do this in the template file for the content type, but that does not seem like the Drupal way (logic in the tpl file). 

I am already using theme_field to adjust the way some of my fields are being rendered, and I have tried to put the logic in there, but I am not sure how to access other fields from within a field-specific theme_field function. Moreover, this does not really strike me as the proper place for this logic either. 

What would be the best way of implementing the different rendering of one field based on whether or not another field has content?
look in to https://api.drupal.org/api/drupal/modules%21node%21node.api.php/function/hook_node_view/7.x

In Drupal 7 yes you can do this in the template file for the content type, -> unless you are changing themes lots perhaps the "custom logic" in the  theme file is the quickest win.