Before beginning a guided fly fishing trip, a sk yourself what is it that you expect from a guide and why you want to hire one.   You need a pre-defined agenda with an accompanying set of expectations.  Clearly identify (in writing if possible) this information -- I'm talking about things like:
“I prefer to fish with dry flies”
‘I'd rather stop and fish out of the boat than in the boat”
“I want to leave late in the morning and return after dark”
“I'm interested in improving my casting”
“My partner is a beginner and I want you to help him all day”
“I tie flies so I’d like a list of recommended patterns”

The client has the responsibility to clearly identify the experience and skill level of each member of the party.  Previous walk & wade or float trips should be mentioned.  If you've never fished out of a drift boat tell the outfitter.  Provide a realistic skill assessment - BE BRUTALLY HONEST.

Find out from the outfitter who is going to be your guide and if that same individual will be taking care of you every day.  Get some specific information about the guide's experience and qualifications.
 
A recommendation on an outfitter from a friend is a great way to make your selection. If this isn't possible, ask for references. Note, relying entirely on someone's reputation can be a big mistake.
 
The outfitter has the responsibility to clearly define what the client can expect.  He should make sure that all client information concerning the trip is provided to the guide.  I like services where the guide is placed in contact with the client prior to the trip.

Good guides are serious about their profession and very customer oriented. Your guide's knowledge will largely determine your fishing success.  He or she should be licensed, insured, endorsed, and certified.  It goes without saying the guide should be prepared to instruct you on how to be a better fisherman.  The guide does not fish because the focus should be 100% on you.

Guides earn a living this way because they love to fish and to help you be successful.  A substantial portion of their income is through tips.  There isn't much of a left over after splitting the take with the outfitter, paying for gas, insurance, and lunch.  Be generous when your guide works hard to please you -- generally this means a tip of 20% or better.