Without supernatural intervention (relevant Divination spells/successful appeals to Knowledge deities), I would rule "maybe," subject to a series of Hard/Very Hard Insight/Intelligence and Perception/Wisdom checks, over several tries for the same subject. (Friendly reminder, in the PHB, Medium DC = 15, Hard = 20, Very Hard = 25; Nearly Impossible = 30). Also, part of the Cant would be the cues to signal a secret message is being sent—and these would just be part of the normal conversation (consider encryption headers wrapped around the un-encrypted message, except in this case, the headers are also intelligible and seem to be the actual message).
Think of TC as being a language where non-speakers have, at best, superficial grasp of the phonetics/syntax, but know nothing of the underlying semantics. Thus, the first check would determine whether the non-Rogue PC has a reasonable shot of knowing about Thieves' Cant (TC's) existence and purpose—maybe a Medium/Hard History/Intelligence check? Alternatively, I would allow a Hard/Very Hard Insight/Intelligence check, to determine something is amiss. Ultimately, the PC can't be actively trying to learn about something he/she don't know exists/is going on.
To actually decipher the message, consider the following: you have just been dumped in a place where you don't speak the language—you can understand the sounds the speakers are making, and can replicate them, but you don't know what the sounds mean. Moreover, the speakers are actively but subtly trying to prevent you from learning the semantics: any attempt on your part to clarify/confirm your understanding of anything is met with deceit and misinformation (so, you would even be led to believe you have the right meaning when you don't). How quickly do you think you'll learn a word or phrase, let alone the language? Probably never. Put differently, we learn languages through heuristics, and part of that process is confirming correct understanding of a term: making the right connection between the symbol (sounds/gestures) and the object/concept (what is actually being talked about). You would need a proverbial Rosetta stone to test your understanding—you need some way of checking the message you think you got against what you think it means. More concretely, unless you hear the same phrases/gestures followed by the same sets of events, you won't be able to decipher meaning: language comprehension is a two-sided equation (symbols on one side, stuff being talked about on the other), if you only ever get one side, you'll never figure it out.
So, I would argue the PC would need to pass a second Very Hard Insight/Int check to have some "grasp" of message from the rest (a sort of placeholder—the PC has heard the same phrase repeatedly, so, it remembers it but doesn't know what it means), then a Perception/Wisdom check to catch the relevant event (and this could be trivial or impossible, depending on the circumstance), and another Int check to make the connection between the message and the event (variable difficulty depending on how much transpired between the message being discussed and the event happening, whether the PC witnessed the event, etc., but leaning towards Hard/Very Hard unless overriding circumstances). After this connection is made several times, then the PC has some understanding of that specific message. Restating from the previous paragraph: it should be nearly impossible to decipher a message without access to the corresponding event. Regardless of your intelligence, no string of sounds/gestures will reveal their meaning on their own.
Finally, I would rule that the Thieves would easily catch someone actively trying to decipher their message (an Easy Perception check); or if the PC is also trying to hide their surveillance, a Perception check by the Thieves against a Stealth check by the PC.
To understand the message, the PC would have to pass:
- A Medium/Hard History check, to make them aware of TC (and start looking for it) OR a Hard/Very Hard Insight check, to help them determine something's up.
- A Very Hard Insight/Intelligence check to have some grasp of the message (the placeholder)
- A Perception check (variable difficulty depending on circumstances) to become aware of the event happening.
- A variable [baseline: Hard/Very Hard] Intelligence check to make the connection between the message and event. After making the right connection between the placeholder message from 2 to the event from 3 at least 2-3 times, then the PC can claim to understand the message.
- Bonus: since the PC trying to decipher the message would need to actively look at the thieves (to catch all the subtleties of TC), the thieves could catch the PC either with an easy (passive?) Perception check or one against a Stealth check by the PC (if trying to be discreet).