When he's the right one, it's easy. You don't have to play any games, second-guess yourself or ever walk on eggshells.
You can have open and honest discussions about what you want out of life (marriage, kids, etc).
He thinks you're beautiful all the time and never, ever qualifies it by hinting that you'll look even better once you hit the gym.
He is respectful towards your family and friends and makes an effort with them. He supports you spending time with them and having your own life.
He's respectful towards his own family, even if they don't always get on.
He is genuinely happy for you when you have a success in anything. He thinks you're brill at your job and dead clever, and never undermines you.
He might not share the same taste in music, TV, films, etc, but he doesn't sneer at what you like.
There's a few pointers I wish I'd had years ago!