At the start of your project, you should fill out a risk assessment checklist and update it monthly. From there, you can compare the results to identify whether any items may derail your project.
A risk is a potential event or future situation that may adversely affect your project. The components of a risk are the possible event itself, the probability that the event will occur, the impact of the event and the frequency with which the event may occur.
Source: Baseline, http://www.baselinemag.com