How can I test Messaging.MassEmailMessage with checking targetobjectids in Salesforce?

Hi,

ou are using "List" just add your email address to the list so you will also get one copy of that mail.
Please check below code (below skeleton for your reference):

Messaging.MassEmailMessage massEmail = new Messaging.MassEmailMessage();
massEmail.setTargetObjectIds(userIds);
massEmail.setSaveAsActivity(false);
massEmail.setTemplateId(templateId);
List<Messaging.Email> emails = new List<Messaging.Email>{massEmail};
emails.add('your email id');
Messaging.SendEmailResult[] results = Messaging.sendEmail(emails, false);

You can change user emailid and you can provide your email id. But your user will get notification about emailid change