If you've read the providers reviews and it mentions msog often, then why not ask, but this can be a ymmv situation.  Also msog can be listed on the ladies web site so check.  Timing of the question can be important. Some guys ask at the beginning of the date. Others ask after the fisrt cup has been spilled. However, ask after the first cup has been spilled, you may get a response "sorry honey not enough time left".  So hobbyist will ask about msog in the beginning of the session so the pace of the date can be set to accomadate a second cup.  For me the second cup is usuallly brought up by the lady as we're laying in bed after the first cup. Sometimes they don't even ask, they just start in on me. While I would say never assume anything about a date, if you book a 2 hr. or longer date, a second is pretty much a sure thing.  Just remember the ladies make the rules.  Good luck.  

Never broach the topic before things get rolling!  I always wait until I am inside her, and then I whisper in her ear, "is it ok if I cum more than once?"  If she says ok, I know how to pace myself.  If she says it will cost more, I hold out and pop 2 seconds before the hour is up to be sure I get my money's worth.  Providers should realize that if you pay for 1 hour, you should get one hour.

From a guy's standpoint I'd wait for the lady to initiate. If you completly wore me out for the first one and our time is almost up I won't get you for the second one. If we can't get to the second one I always say "next time we're going to go for 2!"
Also many ladies do not do MSOG. Make sure yo check out her reviews or website to see if she does.

Don't ask - just act like it is going to happen. Start playing with her. The reviews tell you if they allow. So avoid those that do not provide 2nd rounds if it is important to you.