That's pretty open-ended. Some classic methods might include:
* They intercept (perhaps inadvertently) letters revealing some of this information.
* They seek information about who framed them, and discover information (e.g. letters, scrolls).
* They meet an oracle or fortune teller who gives them clues.
* Someone in the know feels remorseful about how they were treated and tries to give them a way out.
* Someone believes they know something important and tries to get the information from them, revealing the names of the items or other similar clues. Could be someone trying to buy the info, or even kidnapping one or more of them to interrogate it out of them.
* Someone seeks revenge against them and, in a fit of monologing, reveals information that means more than intended/expected.
* They overhear someone talking about it in the tavern.
* They get hired to carry out revenge against the culprits... the patron doesn't realize it's them, but hopefully they figure it out quick!
For your particular items, you might also have them encounter a work of art created using just one or two of the items. Not quite real, but it might have obvious properties, e.g. a moving painting or one usable for scrying, that might get them wondering where it came from. You could even have this work of art be aware and wanting to be completed, but it's not real enough to act on its own (e.g. it's a person who is unable to leave the picture frame), trying to persuade them to help.