Do you have someone specific in mind, Watson? Starts with "Number1" perhaps?
Assuming this is a general question, I would say, you can't tell, you can only suspect. I know 'em when I see 'em. Generally, I do think that persistent apologists have some mental issues, or borderline personality disorders or whatever euphemism you want to use for someone who seems to be a couple of sandwiches and a watermelon short of a picnic.