Re-certification of lifting slings

I need to get my slings re-certified, but am having trouble finding anyone who can do this or how it works. Does anybody know of a company, preferably in the Columbus Ohio area, who does this? The by-laws simply say that all slings must be dated and load rated every three years, but how does someone know if the load rating is sufficient? Do I have to tell them to what weight to re-certify them, if so how do I determine what that number should be? Should it be the estimated boat's weight plus some percent or what? If I don't tell them to what load rate to certify, I have visions of someone just stretching them till they break and then telling me, yep, they were good up to 30,000 pounds!!