Watch the handler at a show and see their rapport with dogs they are working other than their own (clients' dogs)

Talk to others that have used this handler before or have been around them for years ringside - not the references they provide you with because obviously those will be the ones that make them look best

Get a rate sheet and contract and see if important things to you are included (Usually handlers charge mileage and expenses per show, $70 to $80 to handle a show, and boarding - about $7 per day)

Have the handler meet you at your home if you can so they can see you with your dogs and see what you are about

Listen to handler, and truly listen (don't talk at all) about how they work, their plan to train your dogs, including how far in advance they plan schedules and do you enter the show or do they- shows close mid-week so its difficult to plan entries at the last minute, especially if you work, how do they find maors and when do they start looking, start your dog at small shows, how many dogs they have handled recently in your breed - if not many then they won't know which shows and udges are best for your dogs particular look and demeanor, are they currently contracted with someone in your breed - if they are, you'll be last priority to finishing that client's dog, what will it take for your dog to win, how will they train, how long are we looking at etc, how much contact will they keep with you- will you at least know if your dog wins a show that day and what points were attained of do they simply give you progress reports every so often and are THEY actually going to handle the dog or simply bill you and pass the dog to someone less qualified ringside, what else will they bill your credit card for?

Give them your dog, see if they have an immediate rapport with your dog without being heavy handed.  Does your dog like them and are they excited about your dog?  If they are excited your dog will finish at least twice as fast.  If you have a warm fuzzy continue, if not RUN

Let them know what you expect and what is important to you so they know if you are a realistic client, if not then you shouldn't work together or you will only irritate them.  I personally care only if the dog has fun so handlers that go to finish fast no matter the costs are not a good fit for me and vice versa...