Consultants are primarily hired to give advice and counsel--they don't make guarantees that their input will be effective. You are the ultimate judge of their value; therefore, it's extremely important you thoroughly check potential consultants' references. And check more than one. Consultants should give you their resume and a list of clients or references. If they don't, don't hire them.
When beginning your research, don't be turned off by those who consult part time. This can be a good sign that they are active in their area of expertise and can give you up-to-date information.
Interview consultants before hiring one. If they mention companies that have hired them, take notes. At the end of the interview, ask for the phone numbers of the people and companies they mentioned. Again, they should freely provide that information. I also ask them to tell me about a difficult client they worked with. Every consultant should have at least one not-so-great experience. Listen carefully. Their story will give you great insight into how they work. If they claim every job they've had was great, how can you be sure they'll tell you the truth about your issues? If they're overly negative, you should wonder whether they're really concerned about doing a good job.
"When working with a consultant, you should always have an agreement in writing," says Robert M. Chiaviello Jr., an attorney with Baker & Botts LLP in Dallas. "The agreement should include how payment will be made and the scope of the work being requested." It's also very important that the agreement assigns all intellectual property rights to you. Otherwise, Chiaviello warns, the consultant could claim ownership of the idea and leave you without recourse.
A final word about consultants. When approaching them, it's usually better to say you're a "product developer" rather than an inventor. It's a buzz phrase that tells the consultant you're more than a basement tinkerer (even if that's what you are), and they'll take you more seriously.
In the end, using a consultant has few downsides. They'll offer expertise you don't possess, but when it comes to playing the game, they're only a coach on the sidelines. You are ultimately the one who must act.