Ways of saying you think there may be rain.

1- It looks like rain.

The sentence is good English and is used in all versions of American, British ,Australian versions.
It is used when for example you see dark clouds in the sky. 

2- It looks like it’s going to rain.

Strictly speaking ,some people consider it as incorrect or even non-standard because like is a preposition and can not be followed by a clause The sentence,however, commonly used  in informal style. 

3- It looks as if it’s going to rain.

It is used more in both formal&informal speech and in all varieties of English;but it is not more common than the second form.


4- It feels//smells like rain/as if it's going to rain /smell like rain
If there are no clouds, but you have an intuitive feeling or there's an odd wind or a drop in temperature or your bad knee is aching, you'll say it feels like rain. 

You can refer to different aches/pains as signs that it may rain;in this case, you'll say it feels like rain  : ache/pain in knees, feet, back, etc:

 - There's a shooting pain in my back.feels like rain. 

5- It sounds like rain/as if OR like it's going to rain.
We use this form when we have some evidence, for example something we see or hear,
which lets us predict something that is about to happen or will happen soon.

6- It seems like rain/like Or as if it's going to rain .

It is used to indicate that you are quite sure it will rain after you've have heard several weather reports predicting rain .

7- It’s going to rain.
It's used when you're totally sure that it will rain by seeing  black clouds, hearing thunder.

So, the sentences 1-4 mean that you think the sky looks like there might be rain, but you don't know for sure if there will be. 

8-There's rain in the air

The sentence is used even when no rain is literally falling.


9-It looks like the heavens are about to open

It means it's going to rain soon.