When "No. 2" is on your dog's mind, he'll turn into a sniffing, barking, whining, spinning, squatting machine. Well, he might not show all those behaviors, but you can bet he'll display a few.
Your pooch's nose often foreshadows a little potty business. When he's ready to relieve himself, he'll lower his snout almost to the ground, trying to sniff for the best possible bathroom spot. He may also whine, bark or stand by the door. These are all potential signs of bowel movements. They can indicate other things, but if you notice any of these behaviors, take him outside.
Aside from seeing poop falling onto your floor, there are only two imminent signs that your pup needs to use the bathroom: spinning and squatting. If you see these signs, make a sudden noise, such as a sharp "ah," -- do not yell or scream at him -- and quickly put his butt outside. Ultimately, it's best not to wait for any signs. Take him out regularly throughout the day and, if he's a puppy, first thing in the morning, after he eats and before he goes to sleep.