D.N.
Our place will be 100 yrs old in April. We also have ducts. But they are not 100 yrs old. Before going through with having someone come out--poking holes, take the registers off if you can and look inside the vents. Use a flashlight and mirror if necessary. There is a good chance the registers are just filthy from being within reach of everyday things. Ours can get kind of icky with kids but we take them off every year before turning on the heat, clean them off and vacuum the vents ourselves. We flick a flashlight in to see how the ducts actually look. You should be able to also look in your basement if that is where the duct work if. There should be vents down there as well. If they look really nasty, then I would call someone out.