Some providers will show the type of services they provide in their website, such as Greek or rimming. Some will even state that they will only do rimming if the client is trimmed and cleaned from the shower. Besides their personal websites, you can also look at the services listed in their TER reviews, such as Greek or rimming. There is usually an additional fee for these services. You may even read the reviews to see how the rimming experience was with the hobbyist.
Sure, you can ask. But, when its stated, there is no need to. As it relates to recommendations, just find a lady that you like, and check her services. When you arrive, and foreplay is in process, you can make such a request (sometimes with an additional fee).