First of all, I guess I shouldn't say that it's definitely not the Word of God. Rather, I don't believe it's completely the Word of God. So, in order to determine which parts are likely divine and which are human, we can do one of two things. First, would be to focus on what points are constantly focused on. In the New Testemant, Jesus constantly comes back to the idea of loving your neighbor no matter who he or she is, being non-violent and non-judging. These therefore should take precedence over the things that occur infrequently such as condemnation of homosexuality. It is more likely that the small homosexual bits were added or manipulated than the core philosophy of Jesus. Moderates are those that focus on the core beliefs. Fanatics are those that focus on the specifics. Perhaps when we die, we'll learn that the specifics were important. But it is safer to give them less credence if they risk harming someone, which brings me to the second way.
If God is the Creator of all things, this would make Him a force of pure good. Therefore, things that seem helpful in the Bible are more likely to be His Word and things that are harmful are likely to be the products of man. For example the punshments for sin laid down in the Old Testament are more likely to come from the society at the time than God. It would be smarter to simply keep sinners from harming people and let God determine their ultimate punishment. Homosexuality also, using common sense, does not hurt anyone. It is a form of love and arguably not any more harmful than heterosexuality. Conversely, forcing the "sin" of homosexuality down someon's throat can lead to emotional harm and also physical harm if the wrong people are involved.
Therefore it is up to us to judge what is helpful and what is harmful from the Bible and to act accordingly. Are we using our flawed human perception? Yes. But so were the writers of the Bible. We have to actively watch what we do and how it affects people. Ultimately each person must make these determinations on their own. I'm just telling you what my personal beliefs are. I try to avoid anything that causes physical or emotional harm to a person, and try to pursue anything that creates happiness or comfort for a person. The Biblecan be extremely helpful in setting down guidelines to do this. But not always.