Rap... easy enough to look at the entrance. Measure the diameter. You may have to scrape the pipe, carefully, with a knife to determine the type of metal... for instance, lead will appear as black as iron from dirt clinging to the moist surface of the pipe over the years. How old is the house?

I had a building inspection business for about five years and these types of things are of great interest to me.

No, no. Don't run right down to the basement this minute. Just when you happen to have nothing else better to do.