In fact, I would encourage it.
I have done it before, and will do it again if it happens to me in the future.

Choosing a well reviewed provider will help you in this area. Having VIP to read the reviews can be necessary. Many times you will read a review and a hobbyist will state "she is heavier".
When contacting a provider, feel free to compliment her on her photos and ask if they are recent. Some smart providers routinely add new photos and even state when they were taken on their websites.

Before anyone talks about compensating the provider, I will not compensate her if I am deceived.

I had the same question/delimma when I first started. I didnt follow my gut and paid the girl for a session and end disasterous.. Read my review of the event:
http://www.theeroticreview.com/account_manager/viewReview.asp?ID=963236