Hi how can you tell if someone is on a team?

Also, if they have Sherlock Shrubs and/or other team prizes on their island, they are likely, but not necessarily, still on a team.

If you are a president or officer, you will have a list of the people on your boat list to invite. If your team is not full, then players not on a team will have a green button to the right of their name, and you can tap to invite them. If they are already on a team, they will have a gray checkmark, which when you tap on it, the game will tell you they are already on a team. If your team is already full, then everyone one that list will have a gray checkmark regardless, and if you tap on it, game will say your team is full.

That is the only place and situation where you'd be able to tell if a player is not on a team.