A positive family history makes a child more likely to display ADHD, but by no means is it a certainty that this will occur. There are 18 diagnostic criteria for ADHD, and these 18 criteria represent various symptoms of inattention, hyperactivity and impulsivity (children do not have to manifest all of the symptoms to qualify for the diagnosis). It's best not to try to assess this yourself, although the criteria are widely avaialble in books and on the internet. Do some reading by obtaining a book such as Russell Barkley's Taking Charge of ADHD (Revised Edition), see if you think the descriptions are typical of your child and, if so, schedule an evaluation with a pediatric menatl health clinician. Often, particularly with such young children, environmental measures (such as behavioral plans) are sufficient to address the symptoms, without using medication.
My daughter was diagnosed with Adhd and Oppisitional defiant Disorder, and one of the tests they use was the conners scale. Ask the childs school or pediatritian if maybe this would be right for you :)
The Connors Behavior Checklist is one of several commonly used (others are the Child Behavior Checklist, the Behavior Assessment System for Children, e.g.) checklists that contain the 18 behavioral criteria designated in the Diagnostic and Statistical Manual of Mental and Emotional Disorders (ADHD is classified as one sub-type of Disruptive Behavior Disorders) to diagnose ADHD. It is not a test per se, but a list of symptoms. Such lists can be helpful for parents and teachers to quantify what they are observing in the child.