Whichever ones you can't get rid of
Trial and error. Typically, the fact that you are suffering from a malady that an item was meant to prevent was the clue. If you appear to be suffering from a disease even after you kept your new Periapt of Health handy, you might be suspicious.
If they meet an NPC bard and they make bardic knowledge, they know a song or story about the item.
Hello, I have a group that just picked up several cursed items and did not beat the spellcraft to identify them as cursed. Is there any way to tell that the items caused this? Ones like the beserking sword are easy, but a periapt of foul rotting isn't. Is there a way to tell what is causing the curse?
First, you need to know you are under a curse. This usually happens via roleplay, especially descriptive things. Once suspected, some divination might be in order. Once you figure out someone is cursed, you need to determine how long they have been cursed to limit the items likely causing it. Magical re-examination of everything, not just magical stuff, is then in order. Find ways to up that spellcraft roll. The Identify spell is great. Heroism also can help. Aid another can help.
Once you find the cursed item, getting rid of it can be a problem as well. Good parties tend to destroy them. Evil parties try to find ways to give the item to someone else. The spell Beguiling Gift comes to mind. I don't think Dispel Magic works, but Break Curse lets you get rid of the item.
/cevah