With regard to bareback, if they list "ask me" or "anything goes" on their profile, I assume they do bareback with clients (which is not automatically a disqualification for me). With regard to STDs, of course you can ask. The question is how much confidence you should have in the answer.
And if you always use condoms with escorts, why does it matter if they bareback with others. If you are not confident that condoms will protect you from HIV, you shouldn't be doing this.