For TVs and other entertainment devices, reading the specs is the best source of info. A good device will specify the "turned off" power draw. It might be called standby or something like that. EnergyStar improved teh standards a while back and that prompted manufacturers to get better about stand by power draw.
For best results, and LED TV with low standby power would be best. A 40" LED is under 100W operating (probably 60W in practice), and 0.3W in standby. A similar sized LCD could easily be 100-150W and who knows what in standby.
The biggest offenders for remaining on even when turned off are Cable TV and Satellite TV boxes. They typically draw the same power whether they are turned on or off. Turning them off does little more than turn off the lights on the box. I've seen cable boxes that draw 30W 24 hrs a day, 7 days a week, and consume more power than a refrigerator. This continues because the cable and sat companies convinced the EPA to exempt them in the Energy Star rules.
As for plug in power adapters, or "wall warts" as they are often called, the easiest way to tell if it's electronic vs magnetic is to feel it when it's plugged in but not powering anything. If it's warm, then it's just a transformer (magnetic). If it's cool, then it's a switching power converter (electronic). I haven't seen a magnetic wall wart with a new piece of equipment in a long time, but they were pervasive just a few years ago. So any older equipment is likely to have them.
By the way "touch it an feel if it's warm" is a great way to get an idea about other electronic equipment. Feel your cable box after it's been off all night. It will still be nice and warm. Feel your TV too. It should be cool to the touch.