The only way is too test the line. Hook up the antenna in the attic and then test the in question outlet to see if that makes a loop or use a 9v battery and test for the power.

> TEST the unknown outlet for power 1st for safety<

If the cable is from the 70's I would try to rewire it. Once you find out what it is, pull the outlet and look for any markers on the cable to try to date it and see if it is shielded and copper core.