For me, the greatest sign is when you folks would rather spend time with others than with yourselves. And even when you're together, you guys are chatting on phones with other folks.
If you guys can't have real fun by yourselves, someone is definitely getting the fun outside
This is in response to the above question posed on Musings by @aarohi.
I think a lack of laughing together is a good indicator. Not that there aren't plenty of serious times in a marriage and it can't be all fun and game, but if you can't enjoy those moments of laughter and joy, are you happy? Another thing is being easy in your relationship and enjoying spending time together. Couples who are never engaging with each other in their time alone or always need to be around other people are probably not getting what they need from each other.
If you are constantly showing irritation over little things and not showing affection toward each other, that would be another indication that things are not as happy as they seem. If you are constantly having to be be swallowing your real feelings or walking on eggshells to keep a calm atmosphere, things aren't truly happy.
When you are socializing with a couple and they ignore each other or in conversations they are constantly arguing or contradicting each other, then they probably aren't happy. And of course the obvious indication is when either spouse is always checking out and flirting with with other people. It is a shock when a couple that you thought were happy together break up.