I use phrases like "mutual pleasuring", "I enjoy pleasing a woman nearly as much as I enjoy being pleased" and so forth in the introductory email.  

I never have had a provider reply with "I don't do DATY."

but in my experience "more and more" providers are NOT avoiding it.  I honestly can't remember the last provider I saw who didn't allow it.

 
I suppose you could word your inquiry something along the lines of "I like to give pleasure as well as receive it, do you have any rules in that regard that I should be aware of?"

 
I will caution you however that even using that rather ambiguous wording, it  may still cause "some" providers to discontinue the conversation with you.

GaG, I have definitely not seen any slowdown in DATY among providers, but as he also says, explicit language about sex acts is the quickest way to get the door slammed in your face.  When I see indies, I only see ladies with TER reviews, and one of the service fields is DATY (lick pussy), so I tell her that I read her profile on TER and ask "are there any fields on her profile that are no longer accurate?"    Its a soft way to ask what you can't ask directly, and providers are generally okay with answering in that context.  You still might get a surprise at the session with something that is listed that she no longer does, but I think it still tilts the odds of a good session more in your favor than saying nothing.

As others have said, I'm not aware of a trend against DATY.  

In answer to your question,  if really is a concern,  ask discretely as part of booking.  eg, " I really like your profile on xyz ste. Can you confirm that it is accurate?"

This is also an area where recent reviews on TER and backchannel communication are so valuable.

I have no idea why you would screen, agree to meet, fork over the donation just to ask "Is DATY on the menu?"  That is a waste of research tools and probably your time.  Sure if you're dealing with a SW or looking to meet up with someone from a lesser escort forum, but I would hope now that you've for TER that you would thoroughly research the provider to find out if that service is available first.  Emails in general should be nothing more than narrowing down a time/date that both of you can agree on.  There are exceptions I suppose, perhaps she doesn't have reviews.  But even in that case most providers will have some sort of menu in their ad.

the few girls that are not open to DATY usually clearly say so on their site / ad / profile ...  since its something so many ask for if they dont like they want guy to know upfront.     but it is rare amongst any of the higher rated TER ladies.   so stick here and it should never be an issue. ..

of course most girls say 90% guys have no idea what they are doing ...but they let them anyway   LOL

I do DATY just as a "nice guy" gesture.  If providers don't want it, I'd be more than happy for them to tell me so I can let them concentrate on sucking me.