How can I know whether an adjective is an い-adjective or a な-adjective?
Do I simply have to check the dictionary every time I come accross a new adjective, or is there a way of knowing just by looking at the adjective by itself?

Question

Answer

い-adjectives always end in the character い. The い is usually hanging off the end of a kanji, in most cases. Like 狭い for example. But there are some, not a lot, of な-adjectives that end in い. 嫌い for example. If you aren't sure, I'd advice using a dictionary. Though, most of the time, it's easy to guess.