1990 Cadillac Fleetwood Powerless

The only way I can say to differentiate, is to either check exhaust backpressure with a gauge, remove the O2 sensor and drive, or disconnect the exhaust at the connection pre-cat and drive. On a 90 you probably have a crossover pipe from the front to rear manifold, and your disconnect would be at the rear manifold and not easily accessible. I worked on an 89 eldo yesterday that had a y-pipe with a flange before the cat that would be easier. Backpressure should not exceed 3 psi if you measure it.