Yeah, it's tricky to see for sure. I don't pretend to have all or even some of the answers here, but since you asked, and it's a great question, I'll take a stab at it.
First, I'd just make sure that people are happy. If people are honestly happy and enjoying their job, they probably aren't having a problem. Talk to them and figure it out is the most direct way. Indirect ways I would say include:
Social dynamics: Does a team try to exclude a person from meetings, social discussions at work, social activities, etc? Is one person always alone? This is a bit fuzzier since it's about association, and you can't force people to be friends, but it can show a lot about team cohesion.
Work dynamics: I've worked with people who I was almost sure had it out for me, and they would purposefully drag their feet doing work or responding to issues. While this should reflect poorly on the person sandbagging, it's more common that the recipient is blamed for "not getting enough done." Especially with ramping up or helping out, by excluding certain members of the team, coworkers can basically set that person up to fail, while technically doing nothing wrong.
Reviews: Also look out for when people are punished or otherwise told they aren't doing good enough. The measuring stick for this can change based on the person being measured.
Opportunity. Are minority team members given the same opportunities for leadership and growth? Or does a team always give the best work to one or two people that the boss likes, leaving everyone else to clean up the mess? It's hard to prove that you can do the job if they won't let you. This seems to happen a lot in my experience.