According to this resource, some of the early signs of learning disabilities that show up in a preschool child include:
Speaking later than most children
Slow vocabulary growth; frequent inability to find the right word
Difficulty rhyming words
Trouble learning numbers, alphabet, days of the week, colors, and shapes
The core deficit in children with dyslexia appears to be difficulty with something called phonological awareness. Phonemes are the smallest units of spoken language. For example, the words "hi" and "she" both have two sounds or phonemes. Phonological awareness is taught first through oral play with words.
When a young child is tested, it is critical that at least part of the testing focuses on phonological-awareness skills. There are many tests currently in use today that tap this area. One of the most common ones is the CTOPP (the Comprehensive Test of Phonological Processing). A version of this test is designed just for five- and six-year-olds. Other areas that should be addressed in an evaluation of a young child include memory, attention, other language skills, and the common skills associated with kindergarten learning, including identifying the letters of the alphabet, naming colors and shapes, being able to listen to and respond to a story, etc.
You can call the International Dyslexia Association at 1-800-ABCD123 and the Learning Disabilities Association of America at 1-888-300-6710. Both organizations have nationwide branches and offer support and guidance about the evaluation and treatment process. In addition, have a look at two excellent books for parents: Straight Talk About Reading: How Parents Can Make a Difference During the Early Years by Susan L. Hall and Louisa C. Moats and What's Wrong With Me? Learning Disabilities at Home and School by Regina Cicci. Both books have a wealth of information that should help. Good luck!