mibar

I am new to the forum,so excuse me if my post is not entirely correct. I bought a used 2006 Chev., Silverado 5.3,4x4 with 110,000 miles about 1 year ago, I currently have 135,000 miles on the truck and I am wondering,should I replace the plugs or anything else,I have been changing oil and filter every 4,500 miles and added full synthetic oil, I just don't know when plugs need to be replaced. Truck had no service manual when I bought from the lot. So I have no way of knowing the service history prior to my buying it even though it was a lease return from a company so I figured they did regular maintenance.