How to distinguish between homophone words 再 and 在 zài
When zài followed by a noun, it can only mean 在 

When zài followed by a verb, you have to check the context. -- 我在看电影 (I am watching movies) is a complete sentence. 我再看电影 (I watch movie again) doesn't sound like a complete sentence (need more context) 

More example:

我在钓鱼 = I am fishing (complete sentence)

我再钓鱼 = I fishing again  (seems incomplete)

下次我再钓鱼, 会记得确认是不是钓鱼季节 = Next time I go fishing again, I would remember to confirm it is the fishing season or not (it is a complete sentence)