This is always a good question and frequently asked by our clients.
Of course, check the person has the experience or qualifications (if needed) to support you in the role you desire. For example, an accountant will need a level of qualification to submit your accounts and calculate your tax, but what are their skills and experience in providing business advice in general (if that is what you want)?
The main consideration is whether you feel you can work with the person. so go and meet with them, get to know them and take the opportunity to explore their experience and see if what they say is what you like the sound of. Have a couple of meetings with different people and have a basis of choice.
In the end of the day, people buy people and you have to be comfortable with the person you are working with and the advice they are providing. The content can be checked in many ways with the internet, other advisers or professional bodies.