One way, although not foolproof, is look at the stitch count. FSL has many more stitches than regular embroidery. Hugs. Nan W
It can be tricky...some digitizers "think" that their designs are FSL, but do not stitch out well...when I am in doubt, I usually stitch over Organza to be sure it will hold together...
Quite a job you are undertaking. Also you may notice there will be repeating of the outline stitches, stitches will be filled in a cross hatch manner (left, right, diagonal, etc.)An organza overlay would be a good idea if you are in doubt. Have been successful using organza on designs not specifically designed as FSL and on towels with great success. Good sorting.
In my Floriani Total Control Universe, I can load a design in and view it in 3-D. That usually confirms that it has enough cross hatching to be FSL. However, I've decided that only a very few digitizers do FSL well. I have had so many stitchout failures in this area!