The lady should either have a website or an ad that lists required compensation for her time.  I have no experience with providers that do not have the rate posted in at least one of those two formats, in my view stick to the established ladies.

The other issue is that at least some ladies feel uncomfortable with the notion of discussing cash or having it counted out in front of them.  A big part of the hobby is creating an illusion of a romantic tryst, the discussion of $$ shatters that illusion.

Some thoughts

Discussing money with a provider is generally considered rude, because they assume you have done your homework and know what their published rates are, and thus a discussion of money could be construed as your trying to negotiate rates with them.  If you've done your research (their ads, TER, P411, etc.) and you haven't been able to find clear information about their rates, and they or their assistant haven't provided any information while communicating with them to schedule the appointment, then I think it's fine to ask, "Could you please tell me where I can find details on your rates?" or something like that.

Discussing services with a provider before the session is also a bad idea. No one wants a digital trail, e.g. emails or texts, or recorded phone conversations, discussing illegal activities.  (Keep in mind that in Minneapolis and other cities in Minnesota, even a nude massage is illegal.)  Instead, do your research to get a good idea what services the provider offers before your first session, keeping in mind "YMMV" applies.  Then during that session, you'll get better idea what services the provider is comfortable offering, at least in a first session.  The services available to you could open up in subsequent sessions, once the provider knows you're a gentleman.

It's not as black-and-white as some may like, but not much is black-and-white when it comes to intimate encounters between people.

this can be difficult where the provider has not offered any info.  and some times when there is info but her rates have changed.  That happened to me last week, I am inclined to avoid the conversation on the phone if I can read the ter or p411 I will explain "I am a ter person and have read reviews so I do not have any questions unless something has changed"  If no information is available in the add or reviews then I ask "can you tell me what you need for 30 and 60?"  If no information is posted in an add or review and the answer to my carefully worded question is "I do not discuss rates until we meet"  then we will never meet.  

Once at the location the game with $ is you put the $ in a place where she can see it.  Don't mention it or gesture to it and most importantly do not correlate the money with any kind of service definition.  the game is; you did not offer $ for any service and if she is smart she did not offer and service for any amount of money.  In order to bust a lady the blue meanies need to have an agreement to exchange $ for a sex act.  If you place a 'gift' on the night stand with out any comment and she does not touch it then there was no agreement to exchange.  IF... during the body rub she happens to fall down on your dick that could be referred to as things that happen between consenting adults.

If my rambling message makes sense then it is really easy; do not do or say anything that directly connects a sexual service to $.  Refer to a massage or just talk about time.  BTW the same plan in reverse will protect you.  I only responded to a an add for massage, I did not offer or request an exchange of $ for any service other than massage.  Follow that rule and you could avoid a sting.  If the lady ask you to connect the $ to a service, put your $ in your pocket and go home.  

...once you've already completed a session. There's a level of trust established at this point. This shouldn't be asking about rates in order to negotiate a lower rate but to clarify or learn about services for which she hasn't posted a rate, e.g., how much for doubles? Often I've asked ladies whom I've already met what their rates are for 90 min. sessions if they only publish 1 hour or 2 hour rates. I've asked the same question of ladies who I haven't met yet, too; 'Do you have a 90 minute rate?' Nothing wrong with that. Also, I believe, based on experience, that once you've established a good relationship with a lady, it's OK to discuss specific services, face to face, tho.

... to make the lady you want to see feel uncomfortable. Sure, you could have a cagy, guarded conversation in which you never say anything directly incriminating, and there is probably a provider out there who would find it amusing and stimulating to do so, but (a) if you are prosecuted after being caught en flagrante, a prosecutor is entitled to place all relevant evidence before the jury, and a jury could decide that given the totality of circumstances your cagy convo was the final straw that tips the balance beyond a reasonable doubt. And besides, (b) until you get to know her you have no way of knowing if she will be amused or horrified. So just don't. Until you have met and had an opportunity to size each other up, keep your conversation just as vanilla and reassuring as possible.  

As others said, if she simply doesn't give enough info you have no choice but to ask. If her response still leaves you guessing, all you can do is wrestle with the little head to decide if you walk away or roll the dice. You don't get to notice her deposition, place her under oath, and demand answers to your questions.

You also asked if the rule applies 'to all situations or only to conversations with providers that I am not familiar with?' And the answer to that is, very simply, it depends on the lady. With women I like, and who like me, I've had discussions that blow that rule to smithereens. And other women, whom I also like, would be very uncomfortable if I ever broached the subject. It's fair to say that while lying naked and sweaty in bed, many of the normal social strictures are relaxed. But it's still bad strategy to make her feel uncomfortable. So use a little empathy, ease into it, and ease right back out if she's not happy.