Talk about shared plans for the future fade away.
Few shared interests. Not enjoying time spent together.
Little interest in conversation together.
Expressions of appreciation, affection, gratitude and love have become rare.
Nitpicking and criticism are frequent, as is the carrying of grievances over past hurts.
Or one or both might be engaging in passive-aggressive behaviour; covert undermining.
Difficulty or avoidance of talking over differences and/or important events constructively.
Seeing working on the relationship as a waste of time.
The partner no longer seems attractive.
Some attribute may trigger a feeling of disgust or revulsion.
You may see a look or reaction of disgust at certain times.
Loss of interest in and avoidance of physical love-making.
(Careful here; age after 40 can be a factor, i.e., nothing more than the dropping hormones.)
Loss of trust.
Other interests and activities have become more interesting than the relationship.
A future without that person seems much more attractive.
The couple might still be hanging together for practicality and convenience.
Once enmeshed with property, finances, kids etc, separation becomes very difficult.
In old age, many couples feel they no longer have any other options.