What if I used to see prostitutes but I've never told anyone I've dated about it?
That's bad. Why don't you tell your partner, if you really like her?
She's not the right person to tell.
Seeing prostitutes is wrong since prostitution is both illegal in many jurisdictions and also because it is putting your health at risk of sexually transmitted disease. While the past is the past, it isn't right to not disclose your actions to your partners as you might be a carrier for HPV and this could affect your partner. You should get tested for STDs and be honest with your partners if they ask about your past relationships.
Fine. I'm going to tell my partner that I used to see prostitutes and see what happens.
Good it is great that  you are going to be honest with your partner. And in the future you shouldn't see prostitutes again. It would be awful for your partner to have to go bail you out if you get caught.