You can train your dog to tell you when they need to go outside by training them to bark when you get the leash to take them out.
It can take awhile for them to learn but eventually they should start barking at you when they want to go outside.
You can also keep a schedule as to when you take them out such as in the morning and other times of the day and your dog will get used to the routine of when you take them out.
You can also train the dog to get the leash for you when they need to go out.