I ran two identical tests using Dritz Fray Check....WalMart $1.84.

Test set-up:
I used 2 strips of lace about 1/2" wide by 1 1/2" long. I applied tape. Attached each strip to my hand. One strip I designated as the "test strip" (application of Fray Check), the other is the control....no application of sealer.

The test was in 3 phases: light application of Fray Check, moderate application, heavy application.

Light: I applied the sealer to the test strip and allowed it to dry. I then viewed both strips under different lighting conditions and at various angles. The light ranged from direct sunlight, to indirect sunlight, to indoor lighting. In each case I rotated my hand so that I could observe any differences when the lace is viewed at different angles.

Moderate and heavy: Same as above except that the application of the sealer was heavier.

Results:
Light application: I could observe no difference whatsoever between the test and control strips.
Moderate application: No difference in detectability of the lace but I could see a slight increase in shine of the tape.
Heavy application: No difference in detectability of the lace but I could see more shine off the tape.

I then repeated the test with new strips of lace. Same results.

The Fray Check does seem to work nicely on frayed edges of my levis.