How can I tell if my spouse had sex with a prostitute?

That is great that he shared this information with you, that you were tested (and hopefully everything was okay), and that he continues to answer your questions about the event.  It sounds like you are still having some trouble with what happened, and that is okay, and normal/natural.  The question I want you to focus on is why you want to know if he had sex with a prostitute?  I want you to think about what this information would do for you, how you would use it, what-if anything would change in your relationship because of this information.  I also want to ask yourself if you feel like you trust your husband?  Has he done anything like this in the past or since he shared this information with you?  

I don't have an answer for you and you may never know, because it sounds like he might not even know.  He did share this information with you, he did make sure to care for you and make sure you were tested for stds, and he does continue to try to answer your questions.  I'm sure he feels a lot of shame and guilt about this experience and understand that you are still feeling hurt and confused by his actions.  

I think it is important to explore the event, how you are both feeling about it, and even discuss ways to feel safe when he travels again if that is something that is causing stress and anxiety.