How to check if Option key is down when user clicks a NSButton

check [NSEvent modifierFlags]...

if ([NSEvent modifierFlags ] & NSAlternateKeyMask)
{
    //do something
}

Just my 2c, a Swift 3 version:

if NSEvent.modifierFlags().contains(NSEventModifierFlags.command) {
    print("Bingo")
}

One can see the rest of the flags in the documentation for NSEventModifierFlags.