Overall, I don't think there is any way to hard and fast way to tell from grammar. You have to judge from context and common usages.
In the example you gave, it seems a bit vague what (although the usage of 間 to me indicates a change, not a resultant state), so I did a google search and found this paragraph (here):
Having more context makes this much clearer for me, and now I can tell what is going on. I feel this case expresses that the speaker "got sleepy" while on the bus, and then was in a sleepy "state" when getting off the bus, so I would say it is sort of a mix of the two. I would translate the second sentence above as follows:
There were many times when I got sleepy while riding the bus, and there were even times when my grandfather would carry me home.
Here is simpler example where context can help distinguish between the two meanings:
最近、だんだん暑くなってきてるね。 [pretty clearly 'progressive' meaning, i.e. active change in temperature]
暑くなってる。 [could be either progressive, or state]
暑い [if you want to talk about simple state, this is the most common way]