I agree with Ola, mostly its about schedule. My own dog dos not have a set signal, although sometimes she will stand by the door. Mostly, she paces and if I don't pay attention, there's an accident. However, I know her potty schedule and she gets let out at those times so we rarely have accidents.
Some dogs can be taught to use a signal to go out- the potty bells are very popular right now. Be aware though that some dogs will use them constantly whether they need to go or not! They just like to go outside. I've heard of owners getting sick of the constant jingling and just taking them away. And my sister's dog always gets a treat immediately upon coming inside. She's learned to signal that she wants out just so she can come back in and get her treat! When she stays here she is constantly signaling to go out, even if she was out 30 seconds before. Its like a revolving door, in/out/in/out. Makes it hard to know when she really needs to go.
The other thing is that it can really depend on the breed and individual dog. Some breeds are notoriously difficult to housebreak. Hounds and toy breeds come to mind off the top of my head. We had a basset who took 2 years. Honestly, it sounds like you are doing really good for 5 months!
It could be that he just hasn't had the need to discover a cue. He's always been let out on your schedule. If he say, goes near the door, do you let him out? If he scratches the door, would you let him out? It might be a case of waiting for the behavior, and then showing him the consequence. Ie, dog stands by door, you let him outside. Even if he doesn't go potty that time, he'll learn that standing by the door, means you will let him out. The potty bells work on the same principal, only you teach the dog to perform the behavior (ringing the bells) and then give the reward (going outside), as opposed to waiting for an organic signal to occur (scratching the door, standing near the door, whining at the door etc).