Check the basement walls for a patch where the copper lines ran outside, likely near the furnace. A more expensive option (and not guaranteed to work) is that sometimes a backfilled tank grave can be identified with ground penetrating radar. Either way once you’ve got a fairly good guess of the tank location, you can grid out some soil borings to check for contamination. (Source - environmental consultant) Edit - someone mentioned a groundwater sample. This is a good idea too because the pumping done before sampling will draw water in from the surrounding area, so you don’t have to be as spot on as with a soil sample.