Wow! this is a great question, I do not have a clue!

Have you a program that you use to pull up and print or convert designs for the extra details, like Wilcom True Sizer? When I pull something up it shows, the stitch out if it's full embroidery, multiple color stops or just the open weave of the FSL. Another way I tell is if the design is usually done in white or a solid color it is FSL and there is only one color, with no stops. Hope this helps.

A low stitch count might tell you it is not FSL. If you have software that can show the way it stitches out it might help, too. Are there lots of stitches over stitches and linking one part to another? If so it might be FSL.

when I am unsure I embroidery them on tulle, then burn away or cut away around the outside.

If I am not sure, I will stitch the design on organza and burn the excess away. You can just cut it away as well as the threads are so fine.

Most times you can look at the stitch count and figure it out. FSL has lots of stitches compared to a regular design. If in doubt, stitch a sample on organza. Hugs. Nan W

Embroidery is generally a filled design without spaces in the design area. FSL is built from the bottom up by weaving back and forth. I'm attaching a picture of FSL so you can see the weave. If you can look at your design in this way, you should be able to tell the difference.

Well, I see you have your answer...for me if in doubt, I would stitch out over organza or bridal tulle...then burn the outside away...I would also do this even if it were FSL if the item is to be well used...the organza or tulle will add "heft" and stability to the design...

FSL is made up of several passes of stitches so that the threads lock together and not fall apart. The ones I have have stitching vertical, horizontal, and both diagonals. They look like a box with an x in the middle.