Calm it down, guys. Whilst I couldn't agree more with "freedom to be an asshole" with a lot of people, it's more appropriate in a PM than the open forum.

Having posted my own serial story, the method has its own problems and advantages. You get feedback from people straight away on the quality, but things are pretty set in stone. The best way I've found is to keep chapters in reserve, and only post when you're certain of what's going to happen.
That given, it's pretty easy to tell an unfinished story. Usually you can read the chapter titles, or if the Author is good, the reviews, to see what they're doing.

And people do need to lighten up on us Authors, and always remember that just like you're reading it for free, we get nothing out of it than the satisfaction of knowing that people took the time to read, and enjoy or dislike it. Be thankful that you got a part story, rather than none at all. Perhaps instead of being frustrated, you should use the open ending to fuel your own creativeness?
LD