When you are choosing reading glasses over the counter, pick a pair at a strength you think might be right. Then get a magazine and reading it to see if the words are blurry. Then pick a stronger pair of reading glasses and see if it is better or worse. Keep trying on new glasses until you find the best strength for you.
You can try them on and get a book in your hand. If you can see the words clearly and the glasses won't make you feel uncomfortable, then the strength of the reading glasses can fit you properly. The strength is just what you need.
The easiest way to determine the strength of reading glasses you require is to go to an eye doctor. However, you can go to a drugstore and use the eye tests attached to the reading glasses display to see if you can get by with over-the-counter glasses as opposed to a prescription.