Is there a way to find out my team in Pokémon Go?

If you tap your character's portrait in the bottom left corner, your team mascot will be displayed prominently behind you.

As you can see, the Team Mystic logo covers the background behind my character. If you're not sure what your mascot is, then the color of this page should also match your team's color. (red for Valor, yellow for Instinct)

Another quick way to tell is by having any of your Pokémon appraised. Your team's leader will tell you the appraisal results. For instance, when I have my Vaporeon appraised, Blanche tells me the appraisal results:

However, if you don't remember choosing a team, it may be that you're not on one yet. You must reach level 5 before being allowed to choose a team.

Your XP bar fills up as you gain it. The colour of the filling substance is the colour of your team logo (If that helps)

The prerequisite to joining is team is reaching level 5. When you enter a gym, you will be asked to choose a team. From then on, there are several visual cues, such as background or textual coloring, using your team's color.

If you are color blind, I think the easiest way to tell is via the pokemon appraisal feature where your team leader appears.