Ezekiel (7:13), speaking of the impending exile of the Jewish people, says:
כִּי הַמּוֹכֵר אֶל הַמִּמְכָּר לֹא יָשׁוּב
For the seller shall not return to what he sold...
The Talmud (Megillah 14b / Arachin 33a; cited in Radak ad loc) notes that this implies that the Yovel - at which time "the seller returns" to his land - will cease to be in effect, and deduces from this that it was still observed in Yechezkel's times.
Another relevant Gemara, although maybe not as strong a proof, is Gittin 48a, which draws a distinction between "the first Yovel" and "the second Yovel" as far as people's reliance on it actually taking place. Rashi explains that this refers to the first and second Yovelos observed after the Jewish people settled in Eretz Yisrael, and writes:
שכבר הורגלו להחזיר קרקעות... דסמכא דעתיה דמוכר דודאי הדרא ליה וזה בטוח שיחזירנה:
They were already accustomed to returning real estate... the seller therefore can rely on it actually coming back to him, and [the buyer] is certain that he'll be returning it.
This seems to be describing an actual state of mind at the time, rather than a theoretical discussion of how people might have thought.
(The Rambam, Hilchos Bikkurim 4:7, explains "first Yovel" and "second Yovel" differently - according to him they refer to someone who sold his land before (any) Yovel, got it back during Yovel, then sold it again after that. The implication is the same as according to Rashi's explanation, though: at least the second time around, both the buyer and the seller know that it's just a temporary sale.)