I've done it a thousand times, written something that makes no sense, or ommitted a crucial plot point. We've all done it, and the only way to fix what is wrong with a story is to get input from someone else. The best place to find a second reader is among your fellow writing peers. Friends and family may not tell you the truth, but a fellow writer will usually be honest and helpful.
So where do you find someone to critique your work? A local writer's group could work, though personally, I don't have the time required for regular meetings, and if you are in a rural area, finding a group that is made up of more than aspiring authors dreaming of becoming published one day may prove difficult.
Instead, I have found online workshop networks to be very helpful and easy to use. You want to find a closed noteworking site, one that requires users to log-in with a password. This is important because if the general public has access to your work, then many publishers will consider the work to published online, not just workshopped.
There are many sites out there where the critigues are good, and many of the member writers are sucessfully published. You may see references to these sites listed in the acknowldegement sections of book. Like in the book, The Killing Sea, by Richard Lewis. In this book, Lewis thanks his fellow workshoppers on the website Zoetrope for all their insighful comments and help.
I am also a member of Zoetrope, which is free to use, but you must log in to see works. And like most sites, in order for you to post work for critique, you must also review the work of others. Zoetrope.com also has forums and private offices where members can discuss all things regarding writing, publishing, submitting, etc. And it's not just for fiction writing, as their are wings devoted to graphics and art work as well.
Another site that some have found helpful is TheNextBigWriter.com, which allows members to score the work of others, with the goal of winning the work published.
Do a google search for writer's groups, fiction workshops, or fiction critiques and you will be sure to find the one place that does your fiction good. And in the end, isn't that what all writers are trying to do, improve their fiction?