The db queries I used in mysql on moodle 2.2+ follows:
1. find the id of the scale in the mdl_scale table:
mysql> select * from moodle.mdl_scale;
2. For example let's say the id of the scale is 10, in the forum activity it is referred to as -10. Why there is a negative sign I have no idea. So to list the specific courses and forum posts that use a scale:
mysql> select course,name,scale from moodle.mdl_forum where scale = -10;
+--------+--------------------------+-------+
| course | name | scale |
+--------+--------------------------+-------+
| 372 | Forum name | -10 |
+--------+--------------------------+-------+
1 row in set (0.05 sec)
3. In the moodle site as admin, I use the course id (372) in the end of my course site url to find the course. Find the "forum name" in that course and change the scale, so I can delete the used standard scale.
There may be a general SQL query to find all the activities using a particular scale, but I've not tried one yet.