There is no way for you, working alone, to identify true triangulation at FTDNA. You can always contact match X and ask them to search for match Y and tell you how they match each other. But without doing that, you cannot determine by yourself if X and Y match each other.
But you can get pretty close using the Matrix Tool. From the main dashboard, under Autosomal DNA Results & Tools, select "see more" and then select "Matrix." Search for the name of match X (you can use ctrl+f to do this) and then the "add button." Then do the same for Y. The matrix will appear below. You will see blue boxes with check marks if X and Y match each other. The matrix will just look plain, with empty white and grey boxes, if they do not.
So, this tool lets you determine if X and Y match each other. But if you match X on chromosome 1 say from positions 1-10million, and you match Y on chromosome 1 from positions 1-10million, the matrix does *not* tell you if X and Y match each other on that same chromosome. They could share a segment on some other chromosome that they received from some common ancestor they share that you don't, and the matrix won't tell you. Still, in my experience, that is a much rarer reason for X and Y to match than just them sharing that same segment so that you all triangulate.
IMO, the matrix tool is typically "good enough" to get convinced you have triangulation. But according to current WikiTree policy, it is not enough to use as a reason to mark "confirmed with DNA" in a WikiTree parental connection.
