When they lay on the couch, not speaking, not watching TV, not even moving. Just being quiet and close.

If they sigh for no reason, and the sigh is that of comfort and happiness.

As Demeter said, when it seems that the spark is gone, and their kisses are no longer the passionate sloppy kisses with tounges, but small, loving kisses. When the feeling of being together is better than sex. (which is not to say that the sex is bad)