Discoloration of the teeth does not necessarily mean that you have a cavity. Are your teeth hurting you? Are they sensitive to hot and cold drinks or to sugary drinks and foods? I understand that you don't have dental insurance, but to be on the safe side you need to get them checked out by a dentist
who will examine your teeth visually and also with an instrument that can detect if there is decay.
Many people have discolorations on their teeth that may or may not be a cavity. Tooth discoloration is usually caused by extrinsic stains. Extrinsic stains are defined as stains located on the outer surface of the tooth structure and caused by extrinsic agents, such as food stains that darken with time, tea, coffee, tobacco, wine, chromogenic bacteria, and metals, etc. There are certain factors that predispose children and adults to extrinsic stains, including enamel defects, salivary dysfunction, poor oral hygiene, etc. Diminished salivary output also contributes to extrinsic discoloration because it may be caused by local disease, systemic disease, and multiple medications. Other chemical ingredients, such as cetylpyridinium chloride in several mouthwashes, can also induce dental discolorations.
Again, please discuss with a dentist.