Try to find a smith that builds what you want, especially when talking custom guns. Go to matches, ranges, etc to find people who shoot his guns and talk to them.
There are a lot of smiths that are known for specialty work on LR guns, tactical rifles, AR 15s, Short range BR guns, shotguns, stocking, painting stocks etc.
Also, Ideally you want one locally that you can deal with and talk face to face. Now that may not be possible, so go with one who has the references in the specialty area you need.
If you do not have friends or people you know who can vouch for the smith, then go to www.6mmbr.com
and look at their recommended top gunsmiths. They run the range of tactical, LR and SR br smiths. Everyone is extremely well known in their respective areas, delivers a top quality product in a timely manner for a fair price. Their guns are proven match winners and/or game getters. Of course there are more very good smiths, but those have been carefully researched and interviewed on what they do and how before they were listed. They did not pay to get listed either, they had to be recommended by top shooters, and people in the field and be proven to be reliable and fair to their customers.