training to signal?

First, How old is the dog?  Second, don't assume your dog should know.  They don't.  That is why you have to work with them. 
 
Next, instead of hoping he will give you a signal, take him to the spot he usually uses when he goes outside.  Use a word(s) that are simple to understand that represents going to potty.  I have 4 border collie adults and a mixed Border Collie/German Shepard puppy.  I use a phrase "outside potty" to which he barks now.  It took a week or so to get him to bark to go outside, but by using the words along with the action of taking him out to his spot he willing barks to let me know he needs to go out.  Next, I have him "show me potty" to which he immediately goes to his spot and does so.  At that point I praise him and give him a treat for doing so.  Eventually all of it clicks.  My puppy is 14 weeks old and is doing well.
 
It just takes persistence and patience in house training a dog.

hmm, jake doesnt have a spot he usually goes in, he just wanders around til he find a place that looks good to him, squirts out a little bit, walks around to find another place, squirts out a little more, etc.......
 
oh and the dog is estimated at about one year old.

Jaye, first you have to learn HIS signal.  This way you can prevent the accidents.  If he always goes to the same door, that's his signal.  Until you can recognize that, you need to watch him all the time.  If he walks out of the room, follow him.  If he goes to the door and begins sniffing the ground, chances are he has to go.  At that point, ask him your "potty" phrase.  (I use "do you need to go out?")  This will get his attention because he has heard the words many times before when he's gone outside successfully.  If he looks up at you and seems eager, then take him out.  This will help to cement the association of going to door means going to potty.  After a week or so(depends on your diligence and the dog's learning curve) your dog should be able to go to the door and just sit and wait. 

We always make our dogs sit at the door before it opens to go outside.  We had problems recently with our older dog that began to pee more often.  She would just go, instead of letting us know.  Turns out she didn't really know how to tell us, we had just always taken them all out frequently enough.  So now, she patiently sits by ANY door in our line of sight, to tell us she needs to go.  And she'll follow us around if we don't get it.  But no more accidents.

I would teach him to bark or ring a bell.  I think the bell is easiest.
 
Just hang a bell around the door nob about 10 inches of the ground.  Every time you take him out (about once every three hours) ring the bell right before you open the door.  The dog will quickly associate (about 10-14 days) the ringing bell with the door opening.
 
The first time the dog "accidentaly" rings the bell, make sure you hussle tot he door and get it open as quick as you can. 
 
Good Luck...

we taught our puppy to ring a bell. he is about 6 months old now and has been ringing the bell for about a month now. the only caveat i would give is, he rings the bell ALOT. he likes being outside and he will ring the bell just so we will take him out. we have been working on that and it is getting better, but it was driving us crazy for a few weeks. []

edit: i have to say that some dogs will pick up on the bell ringing quicker than 10-14 days. sydney was getting the hang of it after just 2 or 3 days and pretty well had it down pat after about 5 days. we did it just like mic says. bell on a string at the door we take them out, and ring it every time we went out. we also said "potty time". the first few times he rang it on his own you could barely hear the bell ring, but as he got more confident that what he was doing was what we were wanting the rings got louder. now he will ring the bell insistently if he really needs to potty and half-heartedly if he only kind-of needs to potty but just wants to go out.

There will come a time when you will want to ween him off the bell.