Inspect them closely for frayed or separated strands. If there are none, wax it up and keep it on there. I see no logical reason to change out a perfectly good string and then have to spend time shooting in the new string and re-tuning the bow. If you are unsure, take it to your local pro-shop and have them inspect it for you. Most pro-shop owners I have had experience with are honest folks who will tell you if the string is still good and won't just tell you to get a new one so they can sell it to you.

If you haven't shot it in a couple years, you might want to go ahead and take it in to get it tuned up anyway. If you are spending money in their shop, I would think you are even more likely to get an honest appraisal of the string.