Hi Roby,
Young guys are always concerned about this issue. It's not a concern for the married guys...lol. If you have poly on the back and sides it's likely she will detect it. If you go with an all French lace or all Super Fine Swiss lace unit she may detect it but she'd really have to be looking for something. It also depends on your hair length. If you have longer or higher density it's very unlikely she'd feel the base. Now, if you have a unit with a fine mono base (we call them screens and we don't offer them) she will feel it. They're heavy and even with longer hair and good density they're detectable to the touch and often to sight too.
Regards,
John