The general rule for appraisers is to look for similar places that have sold within 1/2 mile of the subject property and that have sold within the past 6 months. Agents do about the same search. When prices first started falling, lenders started requiring appraisers to look not only at past sales but also to look at pending sales and active listings. To establish value of a property I look back more than 6 months as well as look at the current market conditions.
As regards getting a license as an appraiser, you will have to complete a number of college level courses and then take an exam. If you pass the exam you are required to work under an appraiser for a minimum of 2000 hours before you can apply for a regular license.