I think the first sign that your marriage is in trouble is when you ask yourself the question, " is my marriage in trouble?"
The next thing to consider are changes in anything- habits, touch, time together, attitudes, communication, desire to be with your spouse, criticism, sarcastic remarks, irritability, avoidance- all of these little things are signs that there is a problem. The good news is, problems can be addressed and fixed.
Every marriage has times of trouble. Some people think these are signs they are no longer "in love". The truth is, they are signs that your relationship needs some attention.
Of course "trouble" can also mean something more serious such as an affair...but most troubles are more common and typical issues. These issues will take a little work to make things better.