I guess that they attempt to do that by weighing up all the evidence presented to them. Inevitably, they will get it wrong sometimes. However, from the reports on this forum (where, I hope, all the marriages are genuine), it would seem that really genuine cases are rarely denied. However, we all have to suffer that agonising wait while the decision is made.
What I do think is really unfair is the apparent assumption that no one can form a meaningful partnership below the age of 21. I really hope that discretion will be used when applying this rule.