How to spot (if possible) if a final 了 is part of an omitted 要 in a 要...了 construction or simply a past information? There are some examples of sentence 了 that indicate future action which (seems to) have a 要 omitted:

我（要）走了，拜！ - I am going to leave, bye!

I know context usually helps with the issue of time and, apart from that, 我走了 is a pretty common sentence that everyone knows it's an about-to-happen action.

My question is:
If I have any other sentence 了 without any context, for example, "我去中国了", is there a chance that a native listener can think it's a future action with an omitted 要 as in "我(要)去中国了" or does the listener definitely knows it's always a past action?

Thanks in advance!
了 in "我去中国了" does not emphasize it is a past or future action in absent of context.  Consider the following:

我去年去中国了  - I went to China last year
我明年去中国了  - I will go to China next year  

Both are legit and 了 changes depend on the content, althought a native speaker will say:  

我去年去了中国  - I have been to China last year
我明年(要)去中国了 - I have to go to China next year