1. Draw a line through the middle of the inner box, leaving one of two triangles that it may or may not be inside. Ask which side of the line the dot is on.
+------/-+
|  +--/  |
|  | /|  |
|  |/ |  |
|  /--+  |
| /      |
+/-------+
2. Draw a vertical line along the vertical edge of the inner box that extends to the outer box. I'll assume here the dot is on the left side
+--|-----+
|  |--+  |
|  |  |  |
|  |  |  |
|  |--+  |
|  |     |
+--|-----+
3. Draw a horizontal line along the horizontal edge of the inner box
+--------+
----------
|  |  |  |
|  |  |  |
|  +--+  |
|        |
+--------+
4. Now we can determine that the dot must be either inside the triangle or not.