There is no formal and objective way in which to determine if a given country is individualistic. Of course, there is a continuum between pure individualism and pure communitarianism. It is unlikely that any society has ever existed at either end of the continuum. That said, it is clear that the United States is a society that is more towards the individualistic end of the spectrum.
Individualism can be defined as the idea that the individual and his or her rights are in some way sacred and that there should be as little infringement as possible on those rights by the government. When a person’s rights come into conflict with the needs of the society, the person’s rights will generally triumph. The US is clearly a society that holds to this idea in many ways.
As an example of this, we Americans today are extremely inclined to allow people to act as they wish so long as they do not actually hurt others. We allow, for example, people to do things like getting tattoos or piercings that would once have been deemed unthinkable. We have largely done away with the idea that sex should occur only within marriage or that divorce is to be frowned upon.
We are also willing to give in to individualism in more political matters. We allow people to spend as much money as they like on political advertising. We do so because we feel that their right to free speech is more important than trying to prevent excessive influence on the part of the rich. The current debate over gun control can be seen as an example of the individual right to have guns defeating the societal need for safety.
In these ways, it is clear that our society is more individualistic than it is communitarian.