Levis imprints a number on the back of its button. The corresponding number can also be found on the care tag (if present) and is a good way to double check if the imprinted number had begun to fade. Here is a relatively complete source on what the button number means, and where the jeans were made.

Do Levis sizes run small?

Yes! Levis fit perfectly true to size, especially as they use waist and length measurements rather than generalised sizing, making it even easier to get the perfect fit.