Before you join a project, before you even start planning it, it's important to take a moment to make sure it's worth the effort you're about to put into it.
What is the goal of the project?
Is the goal of this project clear? Do you know what your deliverables are? Are the requirements for the project spelled out and agreed upon by all involved?
If the goal, the deliverables and the requirements are not clear yet, the project is not ready. If you start something with a foggy idea of what needs to be done, that project already has serious problems.
Can it be done?
Are the goals, deliverables and requirements even attainable? If you cannot be convinced, in your heart, that you can pull it off, you should not take on the project. If you can't believe in the project, it's not use working on it: you've already set yourself up for failure.
Does the organization really need that project?
Is this a single-client, once-in-a-lifetime project? Is it vital for the survival of the organization? Will it improve the organization's offering/product/opportunities significantly?
There has to be something to gain from successfully completing the project.
Is this project profitable?
It's not just about the money. The question is, what is the organization looking to get out of that project? Is it increased revenues, increased market share, increased credibility in the market?
Once this is figured out, you should know if this project will be profitable. You need to decide what will come out of the project is worth if the effort, time and investment required to make this project a success. It's important to get more out of the project than you will put in.