I find dream checks are the best method of knowing you're dreaming. Dream checks are habits you need to get into to see if you're dreaming. It sounds silly: You should know you're not in a dream, right? The problem is, when you're dreaming, you think you're not dreaming and you have a tendency to not to question that. So, you need to get into the habit of checking if your experience is a dream, and then you will perform your dream check in your dream: If something is amiss, you can say, "Hey! This is a dream!"
My dream checks consist of checking a clock and trying to fly.
Checking a clock: It can be any clock, which I use, but I prefer analog clocks. All you do is look at the clock, look away from it, and then look back at it. Nothing should be unusual if you're awake, -- the second hand might be different, or maybe it is no longer 3:37, but now 3:38. My first lucid dream used this method: I looked at my watch, it was normal. I looked away, and then back at it: It had several small clock faces bunched up near the edge of the watch face. So, I knew I was dreaming!
Trying to fly: This doesn't mean jumping out of trees or anything. All you have to do, really, is tell yourself to fly and sort of tighten your muscles and try to get your body to physically float up. You can do this pretty much anywhere undetected, because you don't need to make any great effort to move yourself to fly. If you're dreaming, you will most likely fly without much effort.
There are many other dream checks, such as, instead of a clock, using words of some kind. This is why patterns could help you: If you start noticing certain patterns while dreaming, you could realise you're dreaming. Or, if you know you can read text sometimes in your dream, then using words to see if you're dreaming may not be the best option for you.
The best thing is to note when things are strange. If something strange happens, you can say, "Is this a dream?" (or even if something strange isn't happening), so that when you're dreaming you will ask the same question, but the answer may be quite different!
I had a dream where I walked out of my room, saw two of my dog Kyra and said, "That's weird, this must be a dream!" And it was. Unfortunately, my dream started itself all over again.
Another time, I saw my grandmother driving my mum's convertible, and I said, "Wait a minute!" That time I became incredibly lucid, could control my dream intuitively (much more difficult than it sounds for me), and decided it would be best to drive my car through several brick walls. (Don't ask.)
The cliché method is slapping or pinching yourself, or asking someone else to do it: This method won't necessarily work. You can actually feel pain and pleasure in your dreams (you know, -- wet dreams are called "wet" for a reason). This is also probably not a very good thing to get into the habit of doing.