Determine Array Key For Field Type Programatically
My solution ended up being this:

$field_structure = field_info_field($field_name);
$keys = array_keys($field_structure['storage']['details']['sql']['FIELD_LOAD_CURRENT']['field_data_'.$field_name]);


This gives an array of all the posible data columns for $field_name, where $field_name is something like 'field_my_field'.