Just express your concern to them thru the door explain to them that you are aware of people posing as cops so others will open up their homes and then getting robbed.
The Investigators will most likely just wait while a uniform is dispatched to the scene for your benefit.
But I think something is missing here that needs discussion.
In the majority of jurisdictions plain-clothes cops won’t answer calls due to this very problem. A plain-clothes officer won’t (just) stop by a house for no reason. If you do get a knock at your door and it’s a plain-clothes cop asking to speak with you it’s almost certainly about something you are aware of, a report you had taken, something you previously called about or something that happened that you maybe witnessed or something in your neighborhood. So he or she will be there about something you previously called about.
Most of our investigators if they are doing follow ups will call you on the phone and will either do it by phone, ask you when a good time to come by will be or just ask you to come by the office at your earliest convenience.
If an unannounced cold call is made its usually made by us, in uniform in a marked car.