I don't normally watch trailers - not even for movies - because I think that there are just two types of trailer, both bad. One is the trailer that spoils too much, the other is the trailer that lies, showing a edited gameplay with lot of cool stuff you'll never do.
Demo? Are them still around?
Usually I just watch the screenshot and the description on the Steam store. If I like them, I give an eye on the reviews (both negative and positive), to see what people like and don't like about the game.
And then the price and, of course, if it's on sale.
It sounds like a long process but it only takes a few minutes, or even less.