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)