His Mannerism:
  1. You are suddenly receiving more attention than usual.
  2. You begin to get a lot of unexpected gifts.
  3. Your instincts are sending you messages.
  4. He becomes very non-communicative.
  5. He becomes critical of your once endearing qualities.
  6. He leaves the room to talk on the phone.
  7. He stops saying, “I love you.”
  8. He acts guilty when you do something nice for him.
  9. He becomes emotionally distant.
  10. He becomes jumpy and easily startled.

Her Mannerisms:

  1. She no longer seems to need your help.
  2. She doesn’t get angry if you don’t spend time with her.
  3. She stops telling you about her day.
  4. She focuses all the attention on what YOU are doing.
  5. Over compensating suspicious behavior.
  6. A man’s voice on the phone.
  7. She seems to cry a lot.
  8. Other people ask you what’s bothering her.
  9. She’s frequently not at home when you call.
  10. She suddenly starts working late.
Of course, not all these are precursors to cheating, just pointers that you might want to take note off which are changes in people's behaviour.