I’m rereading The Lord of the Rings. Appendix A, “Annals of the Kings and Rulers”, tells us that Aragorn son of Arathorn spent part of his youth in Minas Tirith under the assumed name “Thorongil” serving under Ecthelion, Steward of Gondor.
Thorongil often warned Ecthelion not to put trust in Saruman the White in Isengard, but to welcome rather Gandalf the Grey.
Appendix B, “The Tale of Years”, tells us,
2957-80 Aragorn undertakes his great journeys and errantries. As Thorongil he serves in disguise both Thengel of Rohan and Ecthelion II of Gondor.
10th July 3018 Gandalf imprisoned in Orthanc.
18th September 3018 Gandalf escapes from Orthanc in the early hours.
25th October 3018 Council of Elrond.
Saruman’s treachery was not clear to anyone before the dispute with Gandalf in July 3018. And Aragorn did not learn of it till he and Gandalf met again in Rivendell in October. So why was Aragorn already suspicious of Saruman roughly 40 years earlier?
Not sure where this is in the annals, but it says in the Tolkien Companion by J.E.A Tyler
Saruman made his first deliberate move in this direction (toward imposing his will, which was forbidden of the Istari) in the year 2759 Third Age, when he appeared at the Coronation of King Frealaf of Rohan, successor of the mighty Helm Hammerhand. The Wizard brough with him rich presents, and declared himself the friend of Rohan and gondor, and a little later was able to persuade Steward Beren of Gondor to grant him the Keys of Orthanc, the mighty Tower which, together with its fortress of Isengard, commanded the strategic Gap of Rohan. All thought this was a welcome move.
All, that is, except a weary ranger who would see everything given up by Gondor as a challenge to its power.
And it further says that
all the time the Wizard was secretly searching the Tower of Orthanc for a long-lost treasure of the Dunedain … the Palantír of Orthanc.
Then in 2851 the White Council met to think of ways to stop Sauron from coming back
Saruman, hoping that the Ring would expose its location if Sauron were left unharassed, deliberately overruled a strong recommendation (from Gandalf) … that Dol Guldur be attacked.
By his actions, Gandalf may have suspected that Saruman was up to something, although I don’t think Gandalf even knew of the ring.
So, either through his own understanding of the Palantír through the lore of his people or through his association with Gandalf, Aragorn was more naturally suspicious than Gandalf and I think it makes sense that he’d know something was amiss well before anyone else had reason to suspect.
You can read Peter’s answer and all the others at SF&F SE. This entire blog post, both my own writing and the section I quoted from Peter, is under the license CC BY-SA 3.0. Feel free to repost elsewhere.