While each airline may have their own system for numbering flights, it is safe to assume that a flight with an even number is heading either east or north, while a flight with an odd number is traveling south or west.Additionally, flights with fewer digits are typically more popular routes than those that are assigned higher numbers.
Other details, such as the two-letter code that often precedes a flight number, may be easier to determine; a flight number such as "AA702" can usually be assumed to belong to American Airlines, while other major American airlines use codes such as DL (Delta) and UA (United).
Another typical practice is to give returning flights one digit higher than the departing flight. For example, a flight from San Francisco to Washington, D.C. may be numbered 202, while the return flight from D.C. to San Francisco may be numbered 203. Airlines also tend to permanently assign certain flight numbers to certain routes. There are likely to be exceptions to all these rules, so passengers should always double check their flight numbers and rely on the information provided by their airline rather than trying to guess what their flight number might be.