If they are online at odd hours, you should ask them why.
A single man will not have any restrictions on communicating with you however a married man will probably need to be more careful and therefore his responses to your emails, text messages etc may be irregular and erratic.
sometimes things do not work out, in which case you take the high road, try to fix the problems in your marriage, and end things maturely if neccessary. his family will never respect you and you will never respect yourself. Why would you want to be with a man that oes around his own wife and kids backs to be with anothernwoman?
Sounds like he wants to have his cake and eat it too.
Plus a married man is seldom looking for a long term relationship and they believe a casual fling is easier to find on a free dating site. One or two of these reasons alone cannot prove that you're chatting with a cheater.