You look it up in the dictionary. There are a number of kanji lookup methods.
WWWJDIC: Kanji Selection
The "selection type" drop-down lists a number of lookup methods.
Now, if it's on the computer, the easiest way is, within Firefox or Chrome, use the add-on Rikai-chan or Rikai-kun, respectively, and just mouseover the kanji.
I often use my Kanji Learner's Dictionary with the SKIP method, but I also use my electronic dictionary to look some up, too, first. There are, as I said, many different methods.
Now, if you mean without using a reference, you can't really know, but sometimes you can guess at its meaning or pronunciation. Meaning through context. Pronunciation sometimes through parts of the kanji. Oftentimes, parts of a kanji hint at its pronunciation. You will discover this over time, which will help in learning more kanji. I noticed this really started paying off once I knew about a thousand kanji already. Learning the reading of the next thousand was much easier because of this.
