I hate to say it, but mostly by word of mouth.
If someone's good at running and marketing a large business, it doesn't necessarily mean that they do good work, and may mean that they know how to maximise profits by cutting corners whenever they can. That's not always the case, but it seems to be pretty typical.
Instead, look for the small guy who may not have a website and drives a plain van. I work for my dad, and that describes him to a tee, as well as most of the other tradesmen on the jobs we do. My dad doesn't even have a company name, nor business cards. He gets all of his work either through a few regular builders, or by word of mouth from previous customers, and is definitely not a business-man. He started carpentry with his dad after he finished high school, and just slowly grew from there. For most of his life he worked alone, but as they grew older, all of my brothers at one point worked for him full-time (it's just me now, and I'm part-time).
If you do find a potential tradesman, just ask him (or her) how he does certain aspects of the job (ex. baseboard inside corners should always be coped, never mitred), and make it clear that you expect high quality work. If he asks for significant payment up-front for anything other than materials, walk away. Better yet, supply all the materials yourself, and pay a proportion of the job each day or week until it's done. One more factor to consider is price. We charge more than the typical carpenter, but not exorbitantly more. If the price seems too good to be true, it probably is.
I've also found this to be true for auto mechanics. The run-down looking shop is more trustworthy and does better work for a better price than the highly polished franchises.