What is a good indication that they are in or are approaching Liminality?
I don't know. There are many, but sometimes it means something else.
As Liminal Depression approaches, an MLCer may exhibit more anger and lash out more as he tries harder and harder to avoid being depressed.
Some, instead, may exhibit signs of more classic-overt depression.
General Indicators of Depression
Many of these are common to MLC in general--changes in interests, feelings of guilt, hopelessness, inability to concentrate, substance abuse, sleep changes, anger... This is because Replay is Covert Depression and Liminality is Overt Depression. So look for what changes. Many believe that the break up of the adulterous relationship brings on Liminality, this notion is false. I am not saying that it will not bring on Liminality, but many MLC adulterous relationships continue through depression; though the dynamics of the relationship will change.
MLCers find new friends and new activities. Sometimes these become permanent and beneficial life changes, but many of these changes are temporary and will not survive Replay. Is your MLCer still involved with the same new friends or in his Replay activities, or has he lost interest?
An MLCer may display some of these symptoms to his abandoned spouse and others to the alienator.
He may try to hide the fact that he is depressed from his spouse because she may feel that since things are not going well in his new relationship, he will come home soon. Instead, he will show his anger and irritability to her. He may feel the challenge of trying to keep up energetically with an alienator and friends who are much younger and as he tries harder and yet fails to meet his own standards his shame and feelings of worthlessness may surface.
It is important to understand that what you see may not be what others are seeing. You are seeing the act he wants you to see and others are seeing a different act. Few if any are seeing him Be.