No one knows what causes ADHD but structural and functional imbalances in the brain are believed to play a role. Children may show signs of this imbalance early in life. They may be slow to walk and have poor balance because their eyes don’t work well together.

Toddlers and preschoolers may be unable to sit still, follow even simple directions, or control impulses. They may become angry for no reason and hit their peers or siblings. They tend to be impatient, breaking in line on the playground, or interrupting others when they are talking or playing. They may move constantly, jump from one activity to another, and have a high level of energy and a low sense of danger (and perhaps a high threshold for pain). When shopping, they may refuse to sit in the shopping cart or stroller; they may take items from the shelves and open them or throw tantrums if you don’t buy something they want.
Toddlers with ADHD can become over-stimulated and overexcited easily but it can take a long period of time for them to calm back down after this. In situations with a lot of stimulation, they can become impossible to control, hitting, crying or screaming.
Many times, toddlers with ADHD will be up early in the morning and still be ready to play late at night. Some toddlers with ADHD will give up napping or take short catnaps during the day. Sleep problems tend to worsen symptoms of ADHD, causing both parent and child to become irritable.
Impulsiveness is another typical characteristic of toddlers, but magnified in children with ADHD. The toddler with ADHD may jump off playground equipment, jump from windows and run into the street. Toddlers with ADHD cannot be left unsupervised, even for a moment.
Some signs of ADHD in toddlers:

* Inability to sustain attention, even for a few minutes
* Constantly distracted by sights and sounds
* Difficulty sustaining eye contact
* Able to pay attention to certain high interest things
* Able to stay on task in high energy level tasks
* Excessively hyperactive
* Always in motion
* Lack of interest in reading, sitting on laps or cuddling
* Difficulty calming down once overexcited
* Highly impulsive
* Accident prone
* Difficulty sleeping

Its hard for new parents to decide of their child is normal or may have a problem. Mostly because they have not had the support from past family as teachers to let them know if their behaviors is normal or not. We as parents learn from previous generations on how to handle our kids. Without that, many new parents are victims of media propaganda.
Some experts suggest that the first sign of a toddler exhibiting add or Adhd is the inability to concentrate on say playing for a while, like catch or something simple. Or unable to follow a simple thing like “put that down” or come here. They seem distracted. Some toddlers in day care seem to be oblivious of what the day care worker tells them picks fights with others, are careless.
Normal childhood stubbornness seems to appear the same way but eventual capitulation or the ability to make amends or exhaustion can be the difference, A very good article from web MD may help with the concern, the books that are listed below may also be of help.

Disconnected Kids: The Groundbreaking Brain Balance Program for Children with Autism, ADHD, Dyslexia, and Other Neurological Disorders (Perigee Trade, 2009).
Book listing taken from web MD