Most foreign stamps never attain a market value greater than their postage. The value of a stamp is determined primarily by its watermark, the quality of the stamp overall and the rarity of the stamp. Thus, finding the value of a foreign stamp requires assessing where the stamp is situated along each of those three metrics.
Identify the watermark of the stamp. The watermark is the original manufacturer's symbol and can usually be seen by placing the stamp in front of a light. The watermark is largely translucent, but directly applied light will make its presence observable to the naked eye.
Complete a "fast and dirty" assessment of the value of the stamps. According to philatelic expert Harry McColluch, multiply the number of stamps (assuming a stamp is of very fine condition [i.e. not damp, bent, or used]) by 20 cents. That will provide you with a base catalog value of the stamp.
Check the Scott Catalog for the foreign stamps' basic value. The Scott Catalog is the master catalogue for stamp collectors and is available at most public libraries. Check specific stamps for rarity, watermark and corresponding market value, information that is available in the catalogue. Multiply that value by the number of stamps you have of the same kind for a total value of that kind of stamp.
Deduct from the total value of the stamps based on the dampness, general wear and tear, decreasing quality of image and holding condition (i.e. whether they are in a proper folder or not). It will be difficult for an untrained eye to assess how much value ought to be deducted; however, the Scott Catalog also provides a rough set of criteria and corresponding deduction percentage to help you apply grades to the stamps (e.g. a fine grade of X stamp, as opposed to very fine, yields a Y percentage deduction in value from its very fine potential value).