There is no other way of selecting a team than clicking on a gym first. You don't need to be right next to the gym to do this, you can't just click on it from a few blocks away as well. Once you have clicked on the gym, Professor Willow will present the 3 teams to choose from and you can make your choice.

So, in your case I suggest that next time you go to a city, get your cellphone out and click an a gym.

There is no other way of selecting a team than clicking on a gym first. You don't need to be right next to the gym to do this, you can just click on it from a few blocks away as well. Once you have clicked on the gym, Professor Willow will present the 3 teams to choose from and you can make your choice.

So, in your case I suggest that next time you go to a city, get your cellphone out and click an a gym.