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

Unfortunately, you will never know the true answer. This is really frustrating to hear, but you will never 100% be able to guarantee that he has given you the truth. So if you can, work on accepting that, breathing through that,He may not fully remember, he may have lied to you in the past or he is lying now. He may have been honest then or he may be honest not. 

As for options, you can talk to him and provide a safe space for him to tell you what happened. This could be a private conversation with you or at a counseling session. He would need to feel safe to tell you the truth. But remember, there are no guarantees. You would need to make a decision to trust him or not. And go from there. A couples counselor can help guide that conversation, and make sure that he is feeling safe to share. 

More important than if he actually had sex or not, I would evaluate your relationship. Do you trust him now? Were there trust issues before? Does he hide things now? Identifying your current trust levels will be important for having a healthy relationship now. Hope this helps, even if it is frustrating to not have guarantees.

I've been in this situation before and have some insight to share with you.  In my experience, he was lying and trying to cover it up. By pushing you to std testing, he's distracting you so he can shift the guilt he feels and blame it on you. In a guilty man's mind I believe he's thinking-

...if I cheated...you must have as well....but you're much worse than I because it's okay for me to cheat...but not you...now you're a cheating dirty slut...just like that prostitute 

Men believe they're entitled to treat women and children like property. Often they use intimidation and violence to control us.  

He'll never admit to cheating, he'll never apologize and he'll do it again and again. Truly they don't think it's a big deal, it's just their way of being social. They desperately want us to accept it, perhaps join in the act.

It sounds like you've been living with this uncertainty for those couple years since this happened.  

The reality is you won't ever "know" - mostly because of the various versions your husband has provided you.  

Infidelity, though painful, does not have to kill a marriage.  Secrecy and dishonesty certainly will though.   I encourage you and your husband to seek therapy together with someone who specializes in couples work.  The longer you wait to do so, the more damage is done to whatever connection you have and the greater the distrust will become. Please get qualified help, for your and your husband's sake. 

The only two people who know the answer to your question are your husband and the prostitute.

The question you did and can answer is the reason you would doubt your husband, which is bc of his multiple stories.

Ideally your husband cares to restore your sense of confidence in the stories he tells you.

If you state your doubts and your interest for the truth, ideally he will produce enough evidence of the truth so you will have facts and can make sense of them.   Hopefully the two of you will do this sense making together, especially if he did have sex w the prostititute.

If he's unwilling to care about restoring your trust in him, then this is a different problem entirely.

The answer lies in what the outcome will be. How will things end if you find out he had sex with a prostitute? Will you leave? Will you try to work things out? 

The multiple versions are concerning. He seems unable to be truthful and this is damaging your relationship. 

Before considering marital therapy, please find a counselor for yourself. The right therapist can help you uncover what is right for you.