It used to be that a gentleman wouold leave the donation on the dresser as her dressed to leave.  The lady never touched it until the gentleman left.  I have always done this with my ATF, right from our first date.  (I always have the donation ready in a front pocket, no fumbling with the wallet.)

These days, most ladies want the donation up front so they can be sure he's not gonna get service & bolt.  This also makes it easier for LE.  
Some ladies remove the donation at once...  upsellers & under performers obiviously but also ladies who've been ripped off in the past.  
Other ladies, ignore the donation until the gentleman leaves, which is much classier.

There's no way to read a lady's mind, unless she states in her ad or web site.  Sometime the VIP reviews will give a hint.    
I always have the stated donation ready so it really doesn't matter.  There is little risk if you are seeing a well reviewed provider.  There are many threads on this if you look aroujnd here and on the newbie board.

Un/few reviews ladies are always a risk.  (TER VIP is the cheapest insurance.)  You should not see a lady who upsells which you'd know from the reviews.  If you see an unreviewed lady, she can take the money & run, upsell, or just underperform...   and there's nothing you can do except write a review to save the next guy.

Lately, members have seen ads that say "TER reviewed" but if one actually reads the reviews, they are not flattering.  You have to do your homework...  before the date, NOT AFTER.  Take the time to read a couple of pages on each board, especially the terms for newbies.
skb  

I once almost forgot to give it and the lady was so kind not to ask.
After that incident, I give it before, it also eases my mind.

But in the normal situation just as Mr. Fisher says- and always  like Mr. Fisher said the first couple times you see someone.

Once you've seen a lady multiple times- the game can change.  I've written checks, I've paid at end as with a couple of my favs I never know how long we run and how much I owe until it's over.

I've been offered creative arrangements to keep my expenses looking legit- like gift cards to furniture store or home depot if the gal knew she was about to make a purchase.  I've direct depo'd cash into their bank account...  can't count all the ways, don't even want to confess some on a public forum!  But always in agreement with the lady's choice.

It gets wierd though when you pre-pay in creative ways- sometimes you aren't leaving anything at actual visit.  

Then you've got to remember which arrangement you are on, lest you leave and forget to pay!  Luckily haven't made that mistake yet, but  I know one guy who did with a long time repeat.

I prefer to be paid up front in an unsealed envelope preferably discreetly placed in the bathroom or on the table within reach so I could thumb through it discreetly and tuck it away. I am pretty skittish, so I'm not one of those hand me money or fan it out on the table kind of girls. I like discreet unsealed envelopes.

Then again, I also have reviews so people should know I'm not going to rob them, I am who I say I am and I'm not going to have some pimp/driver/crackhead come in with me to hurt you.

If you're worried, stick with the well reviewed providers in your area and you'll be able to avoid these problems.

Katie