Believe me, you'll know! You will see the butt expand to about 2 times normal size. You run your hand down the dog and your fingers get stuck.
Serously though, you should be line brushing at least every 1 to 2 weeks (though I know that some do it ever more often than that). If you go longer, once your pup is past puppyhood, you will have undercoat that needs attention.
Indy's butt is about 3 times his normal size. He definitely needs to be brushed out, then to the groomer for a bath to blow away the rest. Then another brush to brush away the by-blow.
Did I say was looking forward to spring? I think I changed my mind. Muddy/Poopy paws and scraggly dogs. They both look like rescues at the moment.