One or two gable vents on the sides of homes are usually insufficient by themselves. There should be soffit vents at the bottom. Sometimes ridge vents are also used. If the roof is concrete tile then you may not see the vents from the ground at all. The best way to tell is to go up in the attic and look for light coming through the plywood.
Send me pictures of any venting you can find and I might be able to tell you if it is enough.