Viewing a colored object through a color filter affects the appearance of the object. Specific colors of light are blocked by the filter and not allowed to pass through to your eye. By comparing the appearance of the object through a filter to the appearance without a filter, the colors of light that are blocked or absorbed can be determined. This allows one to determine the type of filter being used.
 

The pattern  being viewed through the filter reflects all the primary colors of light towards the filter. There are red, green and blue circles from which red, green, and blue light is reflected towards the filter. And there are cyan, magenta, and yellow circles; each of these reflect two primary colors of light towards the filter. For instance, the magenta circle is reflecting red and blue light towards the filter. And the yellow circle is reflecting red and green light towards the filter.

When you inspect the image of this pattern as viewed through the filter, you recognize that two of the three primary colors of light are being transmitted. The red light and the blue light pass through the filter. As such the filter must be a magenta filter since magenta filters block green light and transmit both red and blue light.