Did you even look at the link i posted?
Using the Winding Number (which i already posted a setup for in one of your other topics) you could check whether a Point is inside or outside of a curve (Winding Number = 0).
If you take a point that is definetly on the mesh island (e.g. the center of a face), all inner curves should have a winding number of 0 and the outer one should be 1 or -1. The setup could look like this:
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