Like Pags and others say, Toothpick test!
For the timing it's all about practice. I know I can put my ribs in at 250 cold and not preheated. I need to start testing for doneness at the 3 hour mark. I like to spritz with apple juice everytime I open the door. (tip I picked up here) and it works very well. They have never been done at 3hrs. But the trimmed up meat from the flap is always a nice treat at the 3 hour mark and just might hold you and your guests over until they are done.