Read the story through in its entirety, not pausing to worry about the historical accuracy. As you do so, examine the structure of the piece: check the technical assembly of the sentences, the smoothness of the prose and the pacing of the story as it moves from beginning to end. Ask yourself whether you enjoyed the story, and what qualities particularly appealed to you. Study the characters and see whether they are fleshed out and interesting. Ask yourself whether the plot maintains cohesion, events occur in a believable fashion and the specifics of the story are dramatic and compelling. These are the elements of all good fiction, and historical fiction needs to include them just as much as any other piece of writing.
Look at the story again, this time focusing on the historical details. They can include specific events (such as battles), the kind of technology available, the things characters eat, the phrases they use, the presence of cultural artifacts such as books or plays and the presence of any historical figures (either as principal characters or as supporting figures). In each case, they need to be accurate to the period; check them against reputable references such as biographies and textbooks to see whether the author got the details right. Minor details can sometimes be forgiven (such as a particular type of cloth characters had worn that hadn't actually been created yet), but major details, such as events occurring out of place or inventions arriving that didn't exist yet, indicate sloppy research on the part of the author.
Examine how the historical details interact with the fiction. They exist as embellishment and flavor, but they also need to vividly invoke the time and place in the readers' minds. They also need to serve an elegant purpose within the story. If Abraham Lincoln appears and talks to the main character, his appearance needs to make sense and serve some function beyond just an amusing cameo. The story should also illuminate the era in which it takes place and give the reader some idea of what life was like back then. The author must have chosen the era for a reason, and the way he integrates it into his overall story is vital to the piece's success or failure.