Detect shapes next to it?

I have a game board as shown in the image below where the player draws triangles in a pattern. If the player draws a triangle where I have marked red I want something to happen, if it creates a triangle of triangles. If a triangle is placed somewhere else I want nothing to happen.

How do I detect that. Raycast or something else I should investigate?

Hi Martin!

How do you create the gray triangles? (by scene editor or by events)
After each gray triangle created (by events or by scene editor), you put (or not) a red point at a position wanted of the appropriate triangle. After that, if the player drawn himself a triangle where there is a red point in this (that is, a collision exists between your not green triangle and the red point), you decide after to do something.

For Xerox: just 5 lines to propose a possible and not tested solution.

a+
Xierra

You could try something like…have three check points for the triangles where the adjacent triangles are and make sure the collission is the shape of the triangle then after each move use
Repeat for each triangle
Point a inside triangle
And point b inside triangle
And point c inside triangle

Once the event is triggered mark that central triangle as done with a Boolean variable