Ok, first of all I'd like to say that I'm not a therapist. This is a personal oppinion.
Second, if communication isn't occurring in a marriage, its best to seek professional therapy or marriage counciling, before calling it quits.
A few signs I know of:
1. Total lack of communication for long periods of time. Avoiding important topics such as: future plans, retirement, vacations, having children, anniversaries, and life changing events.
2. Cheating, flirting heavily with others, and online dating. These are a few that can be grounds for divorce so an obvious choice for someone not willing to file for divorce but still wanting it.
3. Depression, drug use without spouse, and alcoholism. I'm placing these all in one because they are used as a crutch and one or more of these can almost always be found in a bad marriage.
4. Lack of sex or sexual desire. This one isn't an absolute. People often loss interest in sex with age, hormonal changes, and even illness. However, I'm suggesting this as an alternative to cheating as some people feel religiously against cheating and simply won't do it in any case.
These examples aren't facts. All of these things may have reasons that don't point to divorce. That is why I recommend counciling.
Best of Luck,
Jen