1972 dart - stopped running while at 50 mph ? no warning

do you mean a spare spark plug wire and attached it to the tester ? how can i tell if its solid core ? i think what i have , one says magnetic suppression , and another says static suppression mag wire stainless steel on it , yet another just says double silicone 7 mm . how bad would these be for the test ? thanks .
Core wire?
Pull a cap wire off slide boot off, look