jQuery Test if checkbox is NOT checked

Question

I'm having trouble figuring this out. I have two checkboxes (in the future will have more):

• checkSurfaceEnvironment-1
• checkSurfaceEnvironment-2

Basically, I want to write an if statement and test if one of them is checked and another is NOT checked. What's the easiest way to accomplish the following:

if ( $("#checkSurfaceEnvironment-1").attr('checked', true) &&
     $("#checkSurfaceEnvironment-2").is('**(NOT??)** :checked') ) {
        // do something
}