Certified gemological institutes grade lab grown diamonds using the same 4 Cs that are used to grade naturally mined diamonds. It means that the properties of each diamond, such as its color grade, carat weight, clarity grade, and cut grade, are validated by a diamond certificate like the mineral one.
The most well-known grading laboratories that give this type of certificate are the Gemological Institute of America (GIA), and the International Gemological Institute (IGI). You can make an informed decision and compare similarly graded diamonds by having a certificate like these.