You can't, apparently!
You know, I met Maciel, the Legionaries' Founder. This was back in 1993 or 4. This was in Chesire, Connecticut, where the LCs have a seminary. I went for a few days to 'check them out.' I was not sold. But the point is, I couldn't tell that he was a pederast. There was a great number of things I didn't like about the Legion, but pedophilia wasn't one of them per se. They were too inhuman, too money-conscious, too impersonal, too stiff, etc.
I came within 10 feet of one of Catholicism's greatest frauds of the 20th century, and I had no idea. What do people expect - him to have a mischievous grin, drool running down his mouth, big grabby hands, and eyes wandering to and fro, searching for the youngest boy in the crowd? There are a lot of people I instantly don't like. I never talked to him, though. I really had nothing to base a negative judgement on. I did hear him talk, though (through a translator), but nothing really stuck with me. The only thing I remember him saying during these long, translated talks was that he said he used to get by on like an hour or two of sleep a night, but now that he was older he needed more than that. I guess his point was that the seminarians should be depriving themselves of sleep like that too. At the time I was impressed by the asceticism embodied in that, but thought it unbecoming that he shared this detail, which truly humble people do not do...
If I had had a private, lengthy conversation with him, would have I discovered his secret? (Read: secrets - he appears to have had many, mutually contradicting vices.) My guess is no. After all, he pulled the wool over just about everybody's eyes for some 50 years of priesthood. (Although I just learned that he did get in trouble once, a long time ago.) What makes me think I would have been his great unmasker, when greater ones could not, i.e. JPII?
I think that throughout this sexual abuse stuff people have had this idea that the matter is all so obvious that there are only two possible modes of action: either you are with the pedophile or you are against him. If you did not report all actual pedophiles to the police then you were complicit in their evil. Because, truth be told, we are all omniscient. But let's remember, evil people are devious, pedophilia does not have a 'look,' and people are universally owed the benefit of the doubt. Even if eventually you get a sense that something is awry, what do you do? Do you call the police and tell them that some guy gives you the heebie-geebies? That is not an indictable offence!
I rub people the wrong way all the time. In fact, I was 'asked to take a year out' of the seminary because some of the staff thought I must be homosexual, since I was doctrinally strict, had a close friend I spent a lot of time with (who, it eventually turned out, was suffering from SSA - same-sex attraction). But I am not gay, never was, and can't imagine that I ever will be. In other words, the seminary staff had a preconception about what a young homosexual man must be - he must be doctrinally rigid, but they were wrong, and they did wrong to discipline me in this manner. And now, after 10.5 years of marriage and four children - have I proven that I am not gay? Of course not! What is the proof for the non-existence of a condition like this?
Incidentally, this gay friend of mine - after a year of living with him, I had no idea what he was going through. Some homosexuals - you know it. There were a few in the seminary I knew less well than this friend of mine, but I could clearly see what was going on in them. It took an additional 3 or 4 years until I found out about my friend. He told me that he had intentionally hid it from me. Looking back, the only thing that bugged me was that I knew he was withdrawing from my friendship, but I had no idea why. I found out about him when I googled him, looking for his contact info! What a surprise that was.
I guess what I'm saying is, I could have been one of the people that Maciel duped. So could have you.
The real question is, what do people do when they hear an actual complaint of pedophilia or at least have a reasonable suspicion themselves? That is the key matter. Nothing else is. Some bishops failed to act when they should have. Lack of omniscience is not a sin or a crime.