To determine if a child is struggling to learn his letters, you must be aware of what a preschooler should know. Most preschoolers won't be able to print and recognize all the letters of the alphabet, but a four-year-old without any significant developmental problems should be able to recognize at least 10 letters of the alphabet, including those in his own name. He should also be able to print his name. He should also be able to recite the alphabet and sing along with the "ABC" song.
You may be able to tell the child is struggling if she doesn't seem to be learning and remembering any new letters over the course of several months. When you start to notice a problem, show the child a series of letters. Ask her to say the letters names and then write them down. Repeat this with the same letters once a month to see if she's making progress or not. If she fails to remember more letters each time, there may be an issue.
Because children develop at different rates, it's possible a child who struggles with letters may simply learn at a slower pace. There are some other potential issues that could be causing the difficulty. Children who have attention deficit disorder (ADD) or a learning disability such as dyslexia may struggle to focus or retain new information. The signs of learning disabilities vary, but a child who has one may refuse to participate in writing activities or repeatedly write letters backwards even after you correct him.
Fill her environment with letters by buying letter magnets she can play with and hanging up alphabet posters. Point out letters in your environment by saying things such as "Look at that blue sign! I see an 'm' and a 'w.' What do you see?" Take her to the store, and let her pick out her own paper and markers for writing practice so she might be more motivated to work on letters. If she still doesn't show any sign of improvement after a few months, speak to a reading specialist in your area or ask your pediatrician to suggest local resources. Before the child starts kindergarten, meet with her teacher to inquire about getting her extra help at school.