The queston is not an uncommon question to ask. But it may seem difficult to answer. You may not be able to find the answer in the usual places. Sometimes neither the owner's manual, nor the manufacturer's tag on the back will clearly tell you the technology of your TV.
One sure way to find out whether you have an LCD, an LED, or a plasma TV is to simply poke the screen with your finger while it is turned on. If the picture reacts to the poke with a ripple-like effect, then it is either an LCD or an LED TV. Computer monitors will do the same thing. If the picture doesn't do anything, then it is a plasma TV.
Another way to tell is by looking at the surface of the screen while the TV is turned off. Plasma TV screens have a glossy glass surface, and will reflect ambient light (much like the old-school CRT picture tubes did). This means you would be able to see the reflection of bright objects coming in through the window behind you. On the other hand, LCD/LED screens have a matte finish that produces a diffused reflectiveness.
Basically, 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 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.