The United States Patent and Trademark Office (www.uspto.gov) maintains a database of all Patent Attorneys and Agents that are Registered. Check and Verify to see if your Patent Attorney is a Registered Patent Attorney/Agent.
Explore Your Patent Attorney's Qualifications in Drafting Patents in the Desired Technology
Every Patent Attorney will specialize in a certain technology when it comes to patent drafting. Although, many Patent Attorneys are able to draft patents in a variety of technologies, it always helps when your Patent Attorney is familiar with your invention's technology and understands it.
Review some past Patent Applications
Ask to review the work product of your Patent Attorney and see how they have drafted other patent applications. Every Patent Attorney will have their own way of drafting their patents, so make sure you are comfortable in their style and work product.
Ask for Referrals and References
Feel free to ask your Patent Attorney for a list of referrals or references. Many Patent Attorneys will have no problem in providing you this information.
Clarify All Fees and Costs at the Outset
Lastly, discuss the Patent Attorney's fees and costs structure in order to avoid any confusion or surprises during the prosecution of the patent application and during the application process. Some patent attorneys charge a fixed flat rate from beginning to end, while some might charge a flat fee to draft and file an application but charge hourly for any work done after filing etc...
Every Patent Attorney is different so do your homework. The more comfortable you are with your Patent Attorney, the better the relationship and better the end result.