Locate all of the jurisdictions and positions the judge has held. If the judge has only held a single position with a single court, it will be easy. If not, you'll need to track down the judge's employment as well. You can search through newspaper records, or through Internet reports on elections in the past, which should cover when the judge was elected or appointed to his current post and what posts, if any, the judge held previous to the one he's currently in.
Examine the official reports put out by the city, state or federal government that were written by that judge. These reports are put out and published by each level of government, and they give a summary of the opinions and case law that was decided. There, you can find many cases that the judge in question presided over. These published reports can be found at the county courthouse and in law libraries.
Talk to the court clerks. Cases decided in open court are considered public record, and you should be able to search by the name of the judge. If the court has the records computerized, you can search through the database for all of the cases the judge decided on, along with the judge's decision. If not you'll have to look through the paperwork for the cases, which the clerks can pull for you.