dnd 5e – Is there a spell, ability or magic item that will let a PC identify who wrote a letter?

There is a good chance that this will not be available to your players since it is specifically a Knowledge Domain Cleric’s 17th level feature, but it is as far as I can find the most official way to get the information. The relevant portions of the feature are:

Starting at 17th level, you can call up visions of the past that relate to an object you hold or your immediate surroundings. You spend at least 1 minute in meditation and prayer, then receive dreamlike, shadowy glimpses of recent events.

Object Reading. Holding an object as you meditate, you can see visions of the object’s previous owner. After meditating for 1 minute, you learn how the owner acquired and lost the object, as well as the most recent significant event involving the object and that owner.

There is a very good chance none of your players is a Knowledge Domain Cleric, let alone a 17th level one. However, perhaps they could go on a quest to find one and earn their services. A more accessible option may be more appropriate though.

Because the specifics of your situation, it may be possible to use Locate Object to find the other half of the pair of scrolls. This is a bit situational and requires a bit of DM ruling, but I would probably allow it. Here is the relevant text:

the spell can locate the nearest object of a particular kind, such as a certain kind of apparel, jewelry, furniture, tool, or weapon.

This spell can’t locate an object if any thickness of lead, even a thin sheet, blocks a direct path between you and the object.

The DM would have to rule how specific a “particular kind” of object can be, but if you put your own scroll into a lead-lined box then locating another of that “kind” of object should be possible so long as it isn’t too far away.

Xanathar’s Guide to Everything included a section that offers guidance and optional features for the use of various tools that players might be proficient in. For the Forger’s Kit it suggests that the kit (or proficiency with the kit) could be helpful in detecting forgeries, in this case combined with either an Arcana check:

A forgery kit can be used in conjunction with the Arcana skill to determine if a magic item is real or fake.

Or with an Investigation check:

When you examine objects, proficiency with a forgery kit is useful for determining how an object was made and whether it is genuine.

If none of the players are proficient with a Forgery Kit, then this could be another situation where they find an appropriate NPC to deal with. Given the secretive organization they’re working for, they might already be familiar with the sort of secretive person who might have knowledge of forgeries.

Other options

Divination and Commune are spells that allow players to ask questions to a deity, and thus have a lot of flexibility for DM interpretation. Of the two Commune seems more useful, but there’s still no guarantee it would work. Scrying could be argued, though even with the most favorable ruling it’s still a saving throw.

Additionally, it’s common in my experience to allow players to roll a skill check for interesting ideas, even if there isn’t a clear rule saying it’s possible. Maybe the Wizard wants to try to trace the magical connection with their general “sense” for magic honed over years of training. Or maybe someone wants to try to trip up an imposter, referencing some made up history with the contact and seeing how they react. It depends on what you and your players find interesting and entertaining.