It's true of Buddhists generally - that everyone is potentially Buddha. Even people who have done the worst things, the blackest hearts, worst dictators, killed millions of people, they can't kill their own Buddha nature. Well depends on the school of Buddhism, different ways of thinking, only some of them refer to it as "Buddha nature" but it's the same idea.
Then, they say that when someone becomes enlightened - again depends on the school of thinking how they think about it, they see everyone else as enlightened at that point. Which might be hard for us to take, but it is like for us time is very linear - they are doing awful things in this life, and we can see perhaps, that potentially in some distant future life they will be Buddha. But for someone who is enlightened you don't have that same strict linear time sequence and straight jacket concepts and separation like that, so they can see them as enlightened right now.
And that's how they help others by seeing them as enlightened right now. They can still make many mistakes, as ordinarily understood, Buddha made mistakes in the ordinary sense. So - it's not like you search around and try to find someone who is always right, never puts a foot wrong - they might just be obsessive about the Buddha's teachings and not really have a clue about the essence of the teachings. While someone who seems a very flawed being, makes many mistakes as it seems, and does things that are quite crazy at times, they may actually be a Buddha, because Buddhas don't care what anyone else thinks about them, and will do things that help others even if it means everyone else thinks they are crazy. They only care about helping others.
So, we can't judge others if they are enlightened or not. But we can tell if we find them inspiring ourselves, and - inspiring doesn't mean that you are in awe of them and think they are great-what matters is if this is someone who inspires you towards good action and compassion, openness, sensitivity in yourself. Who cares if their life seems to you to be a mess, if they are helping you? And in the other direction how does it help if they seem to lead a perfect life, but you can't follow their example, or apply their teachings in your life, if you have a kind of disconnect between them and yourself? What their life is for them is their problem, or might not be a problem for them if they are enlightened :).