I would use Selenium BuilderSelenium Builder instead of Selenium IDE. If your script was exported as Java, then you just do this:
WebElement checkbox = driver.findElement( By,id('the_id' ) );
if ( checkbox.isSelected() ) {
System.out.println("Checkbox was already selected: " + checkbox.toString() );
} else {
checkbox.click();
}
I use this project template to run my Builder scripts.