Just my observations but I tend to notice that lack of triangulation in the temporal area is usually an instant giveaway. It stresses/draws attention to the hairline, and IMO, contributes to the wide forehead syndrome folks speak of. It would be nice to have fine baby hair for the temples because that's another challenge... having thicker piece hair and trying to blend in with the thinner-diameter hair in the temporal area and the region above the ears which appears finer and of a slightly lighter color. Makes one wanna go full cap and say F trying to match everything up.

With a little practice you "should" be able to attach SFS and be totally undetectable. An exposed hairline will take more time and require reattachment more often to keep it undetectable but that's just the price you pay.