This is more of a personal relationships question than a patent law question, but I'll take a stab at it. The best way is to meet with the attorney in person, on the phone, or on Skype to get a personal sense of the way the attorney operates and if he or she is compatible with you. Then, do some background research! You can look at the attorney's standing with the bar, at websites like Avvo to see what other attorneys say about him or her, look at patents and patent applications he or she has filed at the USPTO website, and so forth.
As far as trusting the attorney with your invention, you could ask the attorney to sign a non-disclosure agreement, however any information you provide to the attorney in the course of inquiring about representation is privileged and the attorney should keep all that information confidential.
Good luck! Retaining an attorney for patent work is an investment of a lot of money and time, and a lot may be on the line, so choose carefully!
You should verfiy that the attorney has experience in the area of expertise you seek. You may want to ask about specific experience in the industry/area that you are in. I assum you are concerned about sensitive information being leaked. Tell the proposed lawyer about your concerns and ask how they suggest your concerns be resolved. In the end, you may want to interview mutliple lawyers and decide who seems the most competent and trustworthy. Don't hire a lawyer you don't trust. Your peace of mind is one of the things that you are paying for in this transaction. Good luck.
When you work with licensed attorneys you automatically obtain certain protections such as confidentiality and attorney/client privilege even as a prospective client. You will not need to enter into any special agreement beyond the engagement agreement to acquire this.
Should an attorney violate that trust you have a cause of action and do not need to ground that legal action under contract law in order to enforce your rights.
If you work with non-attorneys you need to be much more careful because they may not have any fiduciary relationship to you and therefore can get away with a lot more.
Ask for rererrals and you can check any lawyers licensing credentials easilly.
Keep in mind, lawyers are in the business of providing legal services not exploiting their clients' widgets and whatnot. Not only would this ruin a good lawyers business but they are also not positioned to do much about it. That is, while your patent lawyer may know a lot about the underlying technology they are typically not in a position to leverage that knowledge like someone in the field would be. So you have much more to fear from prospective investors, partners, manufacturers, etc.
Most of us here, including myself, offer a free phone consult.
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I agree with my colleagues. Trust is at the foundation of the attorney-client relationship. Personal referrals are helpful, or ask your business or family attorney for someone with a good reputation. Local colleagues can have good insight into experience and reputation of fellow attorneys.
You can check to see if there any claims or client disputes involving the attorney (grievances) as well.
Patent attorneys have all different personalities. Find one that has a bedside manner you like. Meet with them to learn a bit about their style to determine if he/she is a good fit.