I may not be considered ELDER to you all, but I have more than my share of gray hair
There are a few simple tests that you can do to a man to tell if he is otherwise occupied:
A. Does he give you his home number? Not a voicemail, but a number where he actually picks up the phone?
B. Have you ever been to his house?
C. If the two of you are "very close" does he stay over or does he make excuses about why he has to leave?
If he is still married but separated...especially if he has young children, proceed with caution!
And ALWAYS trust your instincts.