For the newbie it is pretty difficult to make a distinction between what is Lithops and what is not.
Just as a guideline I have incorporated this page to help with the clear and clean Identification what is Lithops and what is not, although they may be somewhat related families of plants! No claim is made that all forms within each family are covered.
Plant families that are similar but not Lithops, non exlusive list:
Lapidaria, Dinteranthus, Pleiospilos (split rock), Argyroderma, Gibbeaum, Ophtalmophyllum (Conophytum), Tanquana, Cheiridopsis, Frithia, Fenestraria ... etc
Only similar forms that can lead to confusion are pictured, HTH! Enjoy!