How can I tell the difference between teeth staining and beginning of cavities or superficial cavities?? It's on molars like a brownish line..

Visit a Dentist. After examination and X-rays, your dentist can tell if you have any cavities.
See a dentist. Telling the difference between early cavities and staining is done by a dentist using radiographs and a clinical exam with instruments like a dental explorer, plus their clinical experience. They also take into consideration your overall oral health, dietary habits, etc. With small cavities, it's difficult to do on your own and you can possibly damage your teeth or any dental prosthetics you have.
Not a DIY project. You can't. You don't have the years of training and experience, the instruments and x-ray equipment, the lighting and I tran-oral camera, that a Dentist has. Please don't try to do this yourself. The grooves in your teeth may be merely stained or may be decayed. Be smart, call a Dentist, now.
Professional check. It is not uncommon to have dark stains on molars that can be mistaken with cavities however only a professional exam can confirm that. Of course, if you can brush it off confirmation of being stain.