I just found an interesting article by Gary Direnfeld, a social worker, called "The first few years of school." It's about finding learning problems early on. He says behavioral problems (where otherwise there weren't any) is one sign you child may have learning differences:

Children who surface with behaviour problems early in the school year and in early grades, may have an undiagnosed learning disability that increasing educational demands aggravated. There are clinical scenarios that can quickly lead to suspicion. Children whose challenging behaviour occurs more in groups or unstructured settings, may have an auditory processing disorder. If the medical history includes recurrent ear infections, the risk of an auditory processing disorder heightens. If behaviour presents more during certain subjects or learning demands (visual, auditory, motor), other learning disabilities may be present. Even in the case of a bonifide diagnosis of ADD/ADHD, statistically 50% have a co-concurring learning disability.

If you are noticing behavioral problems in the early grades, here's what he recommends:

A good assessment will be vital to determining the appropriate intervention. Accordingly then, the assessment should look at the child’s developmental and medical history as well as family history and any issues between the parents. Further, the school situation should be assessed to make sure there are no problems such as bullying. Medically, the child should be screened for vision and hearing problems and finally, a psychoeducational assessment to explore for learning disabilities can be in order. A good assessment will help uncover problems contributing to behaviour and academic issues. A good assessment will then lead to proper intervention. Intervention may include special education or at least recommendations to remedy learning needs, couple counselling for the parents, or even intervention by school authorities as in the situation of bullying.

So what are other early signs of learning differences? According to LDonline.org, if your child exhibits the following in his/her preschool years he/she may have a problem:

Speaks later than most children
Pronunciation problems
Slow vocabulary growth, often unable to find the right word
Difficulty rhyming words
Trouble learning numbers, alphabet, days of the week, colors, shapes
Extremely restless and easily distracted
Trouble interacting with peers
Difficulty following directions or routines
Fine motor skills slow to develop

In grades K-4 possible signs are:

Slow to learn the connection between letters and sounds
Confuses basic words (run, eat, want)
Makes consistent reading and spelling errors including letter reversals (b/d), inversions (m/w), transpositions (felt/left), and substitutions (house/home)
Transposes number sequences and confuses arithmetic signs (+, -, x, /, =)
Slow to remember facts
Slow to learn new skills, relies heavily on memorization
Impulsive, difficulty planning
Unstable pencil grip
Trouble learning about time
Poor coordination, unaware of physical surroundings, prone to accidents

To read a thorough summary of studies done about post-institutionalized children and learning differences, please click here.