It is a common misconception that a cavity will always cause a tooth ache. In fact, this is rarely the case. Most of the time, a cavity will not start hurting until it is too late. At this point, the cavity has advanced far enough that the tooth will need a crown, or even worse, a root canal.
The way dentists discover cavities is not usually by the naked eye. Most cavities are discovered through your annual set of bitewing x-rays. If a cavity is present, it will usually appear as a shadow on the x-ray. This is why any dentist will stress the importance of having these x-rays taken.
These x-rays are an important investment in your health because a small cavity is easy and relatively inexpensive to fix. As the cavity grows, it becomes more and more costly to repair. By the time a cavity starts to hurt, it may need a crown or root canal, which is significantly more expensive than a simple filling.
Not all small cavities need to be filled immediately, anyways. With proper care, minor cavities can sometimes repair themselves. So save money in the long run by having x-rays taken annually., and of course brush and floss daily!