Look on the back. On the back of lower front teeth is the most common spot to see it. It is not easy to see without a small mirror and a second to see into the small one. Look along gum lines. If you see a yellowish white crusty material that has ledges and very red gums around it, or the shape is weird, likely you have calculus aka tartar. Go to your dentist.
Look for deposits. Look for deposits along gum line, especially along backs of bottom front teeth, of white to yellow to tan crusty material. You can't get it off, you need a dentist for that. You can't see the tarter that develops under the gum line, especially in the back. You can't see the soft plaque that is the tarter precursor. You can't avoid regular dental check-ups/cleanings. You have to go regularly.