How do I tell whether I have an eye problem or a lazy eye?

Eye exam. Check vision, look at your pupil reflex alignment with a penlight, check alignment with cover/uncover testing, sometimes maddox rod testing if you have one. See if there is a refractive error or if the best corrected vision is limited.
An exam. The only way to determine the etiology of an eye problem is to have a complete exam by an ophthalmologist. During this exam, the need for glasses is assessed as well as the eye alignment and the discovery of any other problems.