They Start Hanging Out With New People
If your partner starts hanging out with people that you are not familiar with you should be concerned. And if they keep you out of that circle of friends on purpose that is even worse. When you try and ask them about their new friends they are reluctant to talk about it.
There is nothing wrong with meeting and socializing with other people but when you do that without your partner it is very suspicious, to say the least. If you become too curious they can often argue with you and some nasty words might be exchanged. At this point there is nothing that you can do, just wait and see what will happen.
If this trend continues for a longer period of time then there is a big possibility that your partner is cheating on you. There are numerous stories from people whose partners started having new friends and all of a sudden the relationship ended.