Identifying if boiler chimney is in use

Hi I have a Worcester R15 boiler in my kitchen. The wide flue pipe curves as an elbow up and out through the wall horizontally and there is an external black flue on the outside of the building. The problem is this flue is positioned through the brick work of an old chimney, the stack is damaged and in need of repair but the roofer has asked us to make sure the chimney is not still in use before he reduces and caps it off. There is no fireplace connected to the chimney and only one flue coming out of the top of it, can a heating engineer advise if it is possible the chimney is in use perhaps from the cylinder upstairs or not?