There is no special qualification. If an attorney is licensed in the state, he can practice any type of law. The best solution is to meet with at least three defense attorneys in person. This will give you some perspective as to whether they seem knowledgeable in the area. You should start hearing the same kinds of things from experienced defense attorneys.
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There is no special requirement beyond aid soon to the bar. But you should inquire about the lawyer's background, including experience trying cases for either prosecution or defense. You need to feel comfortable, and to trust your lawyer. Good luck.
The response I have provided is general in nature, and does not create an attorney-client relationship. My practice is based in Rhode Island, and the law and practice in other states or jurisdictions may be different.
While it may be true that any attorney may be a "criminal defense" attorney, your question asks how can you know if it is not on their web site? In response I would say that if the attorney is not openly presenting themselves as a criminal defense attorney, they may not focusing on that area of law in their practice. What area(s) of law ARE they representing that they practice in? If it is Real Estate transactions, for example, then while they CAN represent a criminal defendant, their focus does not appear to be in the criminal arena. The reason why I call myself a criminal defense attorney is not because I went to some special ABA law school that only lectures on criminal law (there isn't one). It's because that is the focus of my practice, as it is for the majority of attorneys that self-identify as criminal defense attorneys. Chances are then, that if an attorney's website does not indicate that they practice criminal defense, don't list criminal areas or crimes that they represent clients on, or highlight areas of law other than criminal law, they are not likely what you would consider a criminal defense attorney.
While I agree with the others, don't overlook the obvious: Check with the State Bar to make sure the person is a licensed attorney. If they aren't licensed, they aren't any type of attorney!
This answer is for general purposes only, and is no subsitute for specific legal advice that would come from an attorney hired by you, having full knowledge regarding the facts and circumstances of your individual situation. This correspondence does not create an attorney-client relationship, and you should consult with one or more attorneys prior to acting on any of the information provided in this response.