An arranged marriage can be legitimate. The tools we use to distinguish between an arranged marriage, a legitimate marriage in general, and a marriage of convenience include the following techniques. If I may, I will switch to English.
We interview the applicants, interview them together, interview them individually, and corroborate details of their marital life to make sure that they coincide. We may conduct an inspection of the couple's dwelling. In certain rare instances, we may conduct surveillance to determine whether or not the couple are actually cohabiting as they claim to be. If necessary, interviews may be conducted with friends, family, and co-workers.
Additionally, applicants may be required to provide evidence of their relationship in the form of letters, emails, photographs, etc.
Those are the tools we would use to distinguish between the two types of marriage.