How to find which content types have comments enabled?
I searched but I found nothing relative, so I take the initiative to answer my own question. 

MySQL seems to be the answer: 

mysql&gt; select type from comment c, node n where c.nid = n.nid group by type;
+------------------+
| type             |
+------------------+
| my_content_x     |
| my_content_y     |
| my_content_z     |
+------------------+


This can be equally used:

mysql&gt; select entity_type, bundle from  field_data_comment_body group by entity_type, bundle;
+-------------+-------------------------------+
| entity_type | bundle                        |
+-------------+-------------------------------+
| comment     | comment_node_my_content_x     |
| comment     | comment_node_my_content_y     |
| comment     | comment_node_my_content_z     |
+-------------+-------------------------------+


All nodes which at least have one comment:

mysql&gt; select type, n.nid from comment c, node n where c.nid = n.nid group by n.nid  order by type;
+--------------+------+
| type         | nid  |
+--------------+------+
| my_content_x |  115 |
| my_content_x |  90  |
| my_content_y |  521 |
| my_content_y |  295 |
| my_content_z |  127 |
| my_content_z |  11  |
+--------------+------+