Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means stare and the verb vocab version is to stare at (or to look at in a very staringish way).

To learn the reading, just think about how you're looking at something for a lonnnnng time if you're staring at it. In Japanese the word "long" is 長い, and guess what the reading of the kanji part is? It's なが, just like ながめる's kanji-part reading.