Private investigators should be part of any practicing attorney's arsenal. Back before the Internet was the mainstay of every self-proclaimed desk detective out there, there were only limited ways to snoop into other people's business. You could either do it yourself, or you had to hire a private investigator.
Private investigators serve as your personal spies. Before a case is litigated, its almost a given that the parties will try to hash out a deal first. A good PI will flush out hidden assets, find witnesses, photograph accident scenes, and trace names. Information is the key to winning any battle. Perhaps this is why an entire chapter of the Art of War is dedicated to spies.
Ask Your Peers
You could go on Craigslist to find your PI. Or you could ask other attorneys. It's part of the attorney business model to get into people's lives, so smart lawyers will avail themselves of the best in the biz. The problem with sites like Craigslist is that they tend to attract opportunist charlatan types that claim a long list of credentials but have very little experience in finding anyone besides their own mother (see below).
Attorneys have recommended pinow.com, which will help you find private investigators listed in your area. You can also utilize Google and Yahoo, but using the services of actual dedicated agencies or guild sites is probably the best option. Of course, your attorney friends should be able to provide testimonials and reviews of the PI.
Demand to See Their License
In order to be called a private investigator, you must earn a license -- like a doctor or attorney. This is the rule in most states. This license ensures that your PI at least meets minimum standards and will at least help you filter through the riff-raff. Again, Craigslist tends to attract unlicensed weirdos who offer up their services to snoop on targets.
On top of the license, you should also consider demanding additional qualifications from your PI. Certifications like the CFE (Certified Fraud Examiner) and the CPP (Certified Protection Professional) are particularly noteworthy because of their required specialized study. Don't be wowed by any piece of paper: practice due diligence and demand it from your PI.
It has to be said: there's nothing like experience. In fact, this should probably be highlighted. These days, the market supports specialization, so attorneys now have the luxury of avoiding "we do it all" types. If you want a skip-tracer, hire the right snoop who knows how to find assets buried in Antarctica. Shoot for at least a decade of actual experience in the field.
Initial Retainers and Fees
Good PIs can charge about $70 to $100 per hour. This is about the rate you should be looking at. If he charges any less than this, the PI is probably no good. This is especially the case if he is the "renaissance man" of the industry. Good PIs often ask for a retainer of the entire amount of the search. Those that do fall into two major camps: those that can because they're that good, and those who are terrible at their job. Use other means to figure out which one you're dealing with.
Your PI Can't Wiretap
Nice try. You can't hire the PI to spy on your wife after he's finished with your client's case. PIs cannot legally tap phone calls, obtain credit reports, or pull court orders without the target's signed authorization (or court order). You cannot use the PI as your agent to conduct illegal activities.