What is your current relationship with guy A and with guy B?
It sounds like guy B's baby to me; sometimes women who are feeling guilty about sex with the 'wrong' guy will look right at a baby and feel like they are seeing the person's face that they feel guilty about.
So, does guy B know about guy A? Do either know you have a question in your mind? Who is on the birth certificate as the father? What is the situation with child support?
If everything is settled with guy B as the dad, I'd tend to leave well enough alone. But if your conscience is bugging you (or your psyche is), could you tell guy B you'd like to do a DNA test? Are you married to him? If not, you could tell him you'd like to do the DNA test for legal reasons (for the baby's legal rights to his dad to be confirmed).