Don't say anything. It is a pointless question to ask and potentially offensive. Simply choose safe sex practices, and you should not even be concerned about the issue. Safe meaning cbj not bbbj, no daty, no kissing or light kissing only. Or for the truly nervous, choose a non full service option like a bj or hj with massage. You can get over your concerns and nerves if you do non-fs with any sp first, then revisit her when you feel that she has safe practises. YOu can also choose to see an sp who does not provide bbbjs, for example, to increase your comfort level.
I say asking an sp if she is clean, etc is pointless because if she doesn't test regularly, she is going to say yes she is clean. If she does test regularly, she is going to say yes. So pointless. And if someone got test results back 3 days ago, they have still had experiences that may expose them to an std by the time you visit with them.
Chances are lower with sps tho because they are aware of transmission and do take fewer chances with their health. You are better off with a sexual encounter with an sp than some random chica off the internet dating sites, or a nightclub, in other words.