Go over and knock on the door, or call and ask if everything is alright. Try to be really polite and nonintrusive even if they are rude to you- that way, they are less likely to think of you as interfering or critisizing their parenting skills. Say something like "I'm concerned that [name] has been crying- is there something I can do for you? Maybe watch him for a little while?" They might still be rude, but this way, it's posed as concern and an offer for help rather than a critisism.
If this happens often, you should call your local child protective services (listed online or in the phone book). It's normal for children to cry sometimes, even over trivial things or for no reason, but an hour seems very excessive. A child crying for an hour is an unhappy child. He is hungry, thirsty, or has another need that's being denied.
Be curious but really that could be for a lot of reason. Ear ache, spoilt behaviour, training to sleep in his own room etc. I would still be very curious as child abuse is so common that I myself would find it hard to let it go. Look for marks or emotional behaviour when u see him. Hope all is well.
You should ring the bell...and make sure you bring your cell phone..just say that you heard the baby screaming and wanted to make sure everything was alright.if you still think something is going on..call the police.
You can go check on them, sometimes it can be that he is trowing a trantrum or wants something and stuff like that. If not, call the police so they can check on him?
That does seem like a long time for a child to be crying, I would go over and inquire if all was well, and if they don't answer the door I would call the Police.
Go see if thr ok, they may need help