The thing about isolating motors is both a misnomer and nothing to be afraid of. Most engines are not isolated from the frames, What you want to make sure is that the motor poles are isolated from the frame. You can do this by putting one lead of your multi-meter on the and the other on the axles of your trucks.
You should at least check this on all decoder installations that are not plug and play. More often than not, it is not an issue.