Its hard to explain in writing, but the lace presses down in to the glue and is no longer visible.
Properly attached, the lace isn't what would give a hair piece away.
Think Beyonce. All her hair styles are lace front wigs.
What will usually give a hair piece away will be:
A mismatched colour/texture.
Hairline too low on the forehead.
The piece having way more hair (density) than your natural hair at the sides & back.
A poor haircut.