Any truck stop will do it for $10 or so. Start to worry when it feels unsafe. I'd be worried at around 6000 lbs with the stock suspension, but with an aftermarket I feel safer pushing the max gvw. I've run modified trucks (Toyota and others) at 40% beyond their max gvw without issue in much harsher environments and conditions than anyone here has put their personal vehicles in. It shortens the life of the truck of course, but they still perform. I think it's a fairly lofty proposition to get an exact worry number on heavily modified vehicles, though.