Just because you've purchased that nice new HDTV for your living room doesn't mean you automatically have HDTV quality in all your television viewing. The HDTV label on the new display tells you that the set is capable of receiving high-definition digital television signals and presenting them to you in their fullest quality. However, not every signal the set receives will necessarily be HD.
So how can you tell if you're watching HDTV? There are two things you'll need to check: the kind of signal you're receiving, and how you're getting that signal to your TV.
First, check to be sure the original video is in high definition. In broadcast TV, if the picture has filled the entire screen of your HDTV set, and it doesn't look stretched to fit, then you're probably watching HDTV. Some channels dedicate themselves to being "HD," but be wary of this label. Many "HD" channels have HDTV broadcasts when available, but also show standard definition (SDTV) shows, filling the extra screen space with vertical bars. Other HD channels are committed to only providing programs originally recorded for HDTV. Besides broadcast TV, digital media such as Blu-ray players can also provide high-definition video.
Second, check that you're receiving the video as a digital signal. This is automatic for digital devices such as Blu-ray players. For broadcast television, though, this means using a digital antenna, digital cable TV or digital satellite TV. Digital cable typically requires an upgraded service and a special receiver from your cable provider. A digital antenna receives over-the-air digital broadcasts (DTV), but your HDTV may or many not have a built-in digital antenna and tuner.
Though these two checks are sufficient to determine if you're truly watching HDTV, you can go one step further just to be sure. This extra step is to use a digital connection (cable) such as DVI or HDMI between the video source and your TV. If you use analog connections, such as component video, you may or may not notice the improvement in quality.