If the girl covers your bum/privates with a small towel, then it is probably best to be cautious in how you ask. A girl will usually give you hints that extras are on offer, such as suggestively running her finger nails along your body. One swipe of the erogenous area may be accidental, repeated swipes is highly unlikely to be, and a good sign that extras can be asked for. If the massage is done with your todger on full display, then extras are almost certainly on offer. Exactly what extras are on offer is of course another matter, but only once have I had a massage where at least a basic HE has been refused.
If it is your first time there, usually not best to ask upfront, as she may think you are undercover police or such like out to trap her. But if you have seen her before, and have had extras before, then asking upfront can save that awkward break in proceedings where you have to go and dig out the extra cash. Extras should always be negotiated discretely with the lady in private, never with "the management" present.