If you still have the owner's manual that goes with your television, it may tell you the kind of TV that you have - or it may not. You could look on the back of the TV and read the manufacturer's tag - but that may not tell you either.
One sure way to know whether you have an LCD/LED TV or a plasma TV is to poke the screen with your finger while watching TV. If it is of the LCD/LED variety the picture will go slightly dark where you poked the screen reacting with a ripple-like effect, and then quickly return to normal (computer monitors do this as well). If the picture doesn't do anything when you poke the screen, it means that you have a plasma TV.
Plasma TV screens have a glossy glass exterior that will reflect ambient light, much like the old-school picture tubes. For example, with a plasma TV you would be able to clearly see the reflection of daylight coming in through the window behind you. On the other hand, LCD/LED screens have a matte finish and are less reflective. This reflectiveness, or lack thereof, is most noticeable when the TV is turned off.
Another way you can tell the difference between these two technologies is to observe how bright the picture appears across their “viewing angle”. As you walk across the front of a plasma TV while watching the picture, you will notice how it will have the same brightness no matter from what angle you are viewing. LCD/LED TVs are the brightest straight on. High-end LCD/LED TVs have almost as good a viewing angle as do plasma TVs. Curved LED TVs will be the brightest toward the central viewing point.
An LCD or an LED TV is nothing more than an overgrown computer monitor with a built-in tuner and a bunch more jacks in the back. The LCD display is constructed of a liquid crystal matrix layer with an array of cold-cathode fluorescent (CCF) backlights behind it. The backlights shine through the LCD layer to produce the picture.
LED TVs are basically of the same technology as LCD TVs. The major difference between them is in the display itself. An LED display panel is, in fact, an LCD panel with white light-emitting diodes (LEDs) as backlights instead of CCF backlights. Consequently, LED TVs are thinner than LCD TVs, and the "whites" on the LED TV picture are whiter and brighter. This is because LEDs are smaller and brighter than CCF lamps.