The first clue is his name. "Shiftlet" would mean "allowing one to shift" (change, not be reliable), and is close to "shiftless," which implies a kind of low greed.
The old woman's reaction is another clue; she sees him as a tramp.
O'Connor also provides clues in her description of Mr. Shiftlet; he has a "jutting steel‑trap jaw," and a steel trap would betray people, clamp down on things and not let go, holding them tight. Later, his gaze is described as "sharp," which is, among other things, not friendly.