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.