In "Hearts and Hands," O. Henry is showing us how easy it is for appearances to be deceptive. In the story we're presented with an upper-class lady who recognizes a past member of her social milieu. Naturally, she automatically assumes that, as he's handcuffed to a rather scruffy-looking man, he must be involved in law enforcement. Miss Fairchild simply cannot comprehend how the well-dressed, well-spoken man she used to know could be anything other than respectably employed.

Although Miss Fairchild is horribly mistaken in her assumptions, she's not the only one to be taken in by Easton's smooth, charming exterior. After all, that's how he managed to get away with being a con artist for so long. And it's because of his less-than-respectable profession that he's now sitting right opposite his former acquaintance on a train, right hand shackled to a marshal, on his way to Leavenworth prison. Although he may have been apprehended, Easton's still able to use his dapper appearance to deceive.

"Hearts and Hands" is a wonderful example of O. Henry's signature literary style of irony.  It is like watching a magician performing slight of hand. 

You are presented with three characters: a U.S. marshall, a fugitive, and a proper young lady.  They have a chance meeting on a railroad car. 

The young lady goes to shake hands with the younger of the two, for she recognizes him as a former acquaintance. He shows great regret at the need to take her hand with his left hand, as his right it tethered to that of the grizzled older man. It is her assumption that her well-mannered, stately friend is the marshall, thus leading to the assumption that the glum, time-worn half of the duo is the fugitive. 

It is only as the two exit to the smoker's car that another passenger points out the obvious.  A marshall always keeps his right hand unshackled. 

It is at this point that O. Henry quite artfully points out that you cannot judge a book by its cover.