I have a brick chimney with two exhaust ports. One goes to a fireplace. I am wondering if there is a way to determine where the other port goes to see if I can use it for exhaust for a furnace.
Have you had your chimney cleaned lately? If not, call a chimney sweep and ask him about it while he's doing his thing. In most cases, using the same chimney is a no-no. But if your chimney is real big and has two independent ducts, maybe you can.
Have you explored the basement? There's only a limited number of places where the other flue (I assume that's what you mean by "port") could go.
If you can't do it, a sweep should be able to tell you where it's going. If it's separate from the fireplace, you could get it relined and potentially use it for a furnace, if the size is right.