Well...

Trust your instinct.....better to walk away before there is an issue as opposed to later on tell yourself----Damn---"I knew there was something wrong about this deal, should have gone with my Gut"!!  In this business or activity, you have to make instant decisions---if you take time to way all the possibilities---which you should have already done as pre-work....all you have left is your intuition ---follow it !

So it's more based on expectations that are met, or not, rather than just a first instinct. Does the girl match her photos closely enough? Is she dressed in a manner you were anticipating or reasonably close? Does she greet you with the friendliness you heard about and experienced through email/text? Is her room clean and inviting?  
.
The above can all be ascertained within a minute or less so at that point, if things go as planned during this intro stage, I feel very comfortable about how things will transpire over the next few hours.

first Instinct's with first impressions.  Instincts cannot be quantified and are rather ethereal, so any decisions resulting from so-called instincts are no better than an educated guess.  First impressions are based on information and facts which cannot be ascertained until you have met in person.  I can be well into a session before I form a first impression based on looks and services, which is the TER standard for providers.  With that said, I will not continue a session with a provider whose end stinks.

nothing rarely is! Id the gut saying "hmmm.... not quite up to par"?? I go with it!  

But- Really sketchy location, pics NOT even a decent attempt, too many "instructions"? I don't follow through! I've given the lady 100 just for her time, leaving her at the door. Yahoo/Google maps have stopped plans right in their earliest formation!