Speaking from experience ( I was in Federal Prison ), there are many ways inmates find about others crimes, but to start I have to tell you there are “phases” of incarceration, pre-sentence and sentenced. Pre-sentenced as the name says it you are in jail, not prison, waiting for your date, in there it is pretty hard to know what others have done because usually people keep it to themselves, because either you have commited a crime like rape or molesting, or you are part of a conspiracy to distribute drugs and do not want to give info to anybody that could be a potential snitch. They are everywhere. Usually though, most people have no problem saying they are in jail because of drugs. But not the details. Now, in jail like I said is very hard to know who are the child molesters or rapists ( very few federal cases as well ) but for some crazy reason the ones that are for that kind of crime they will usually say they are in for white collar crimes or something to do with computers, always very vague. So if we detect that the ones that say are in for white collar and have visits from wives, kids, family etc, they are usually not lying. But the ones that do not have visits or only the mother does, usually those are the rapists, and in some places are told to check in ( go to solitary ) or will be punished, even though they might be clean, or if there are a considerably number of young inmates they will be attacked even if there is no concrete evidence to say they are guilty of rape or molestation. Some places is crazy like that.
Now in prison, it used to be that once you got there, you would be approached by one of your race, first to see if you where on a gang and second to know what you are in for. So you would have to show your papers ( PSI, pre-sentencing investigation ). In these papers there is everything people need to know about who you are and details of the crime. So if you where a snitch, or molester you would be in a world of hurt, so authorities began to lock non-desirables in the hole to prevent these attacks. But few years back ( 2002 I think ) the authorities realized ( with the help of overcrowding ) that inmates having that paper was a bad idea, and in order to liberate people in the hole for inmates that where rowdy. So now people can’t ask you for papers, but there are other ways to find out, but usually are used if there is a good reason of suspicion, and that is paying a guard to provide you with the info, or somebody in the outside investigate the case of another inmate ( if you are in a gang there are some other ways too ). Usually though rapists are asked before they put a foot in general population if they want to be there and risk it. Most don’t.