[Answer]-Which is better practice, checking object exists in Manager or Method?

Use first() instead of get() so no exceptions will be thrown:

Manager:

class CustomerReferralManager(models.Manager):    
    def get_referral(self, mobile_number, campaign):
        return self.objects.filter(mobile=mobile_number).first()

Method:

referral = CustomerReferral.objects.get_referral(mobile_number=mobile, campaign=campaign)
if referral:
    referral.complete_referral()