"The Fox and the Forest", a short story by Ray Bradbury.
ISFDB synopsis:
A couple escape from a dystopic future into contemporary Mexico and attempt to evade discovery by agents of the future seeking to return them.
Excerpt:
They were in Mexico in the year 1938.
[. . . .]
Susan and William sat and ordered a drink.
The stranger was examining their clothes, their hair, their jewelry—the way they walked and sat.
"Sit easily, said William under his breath. "Look as if you've worn this clothing style all your life."
"We should never have tried to escape."
"My God!" said William, "he's coming over. Let me do the talking."
The stranger bowed before them. There was the faintest tap of heels knocking together. Susan stiffened. That military sound!—unmistakable as that certain ugly rap on your door at midnight.
"Mr. Roger Kristen," said the stranger, "you did not pull up your pant legs when you sat down."
William froze. He looked at his hands lying on either leg, innocently. Susan's heart was beating swiftly.
"You've got the wrong person," said William quickly. "My name's not Krisler."
"Kristen," corrected the stranger.
"I'm William Travis," said William. "And I don't see what my pant legs have to do with you."
"Sorry." The stranger pulled up a chair. "Let us say I thought I knew you because you did not pull your trousers up. Everyone does. If they don't, the trousers bag quickly. I am a long way from home, Mr.—Travis, and in need of company. My name is Simms."
"Mr. Simms, we appreciate your loneliness, but we're tired. We're leaving for Acapulco tomorrow."
"A charming spot. I was just there, looking for some friends of mine. They are somewhere. I shall find them yet. Oh, is the lady a bit sick?
"Good night, Mr. Simms."
They started out the door, William holding Susan's arm firmly. They did not look back when Mr. Simms called, "Oh, just one more thing." He paused and then slowly spoke the words:
"2155 A.D."
Susan shut her eyes and felt the earth falter under her. She kept going, into the fiery plaza, seeing nothing.
