By the quality of your work. You can be a solo artist just starting out, but if you have a couple dozen logos that you have designed, a client will be able to see your style and your professionalism. You can redesign existing logos, or make up your own companies and brands to illustrate. You can show logos you design for competitions, such as DesignCrowd.com. Logos pretty much speak for themselves when it comes to "professional" looking...
By looking at their record of work, including asking for references and contacting them.
A "good" design agency will be able to reassure a client that they are professional by the attitude and approach they take to them and their work.
btw a company that calls itself a "logo design company" is probably not a fully developed agency – logo design is one aspect of corporate identity which is a much wider discipline – not every company will need a full identity but they do need to know that their logo is not the only element considered.
Portfolio, awards/other types of success, degree/other type of education or background in the field, customer service, duration of experience in the field, price (is it reasonable?), reviews...
Answers & Comments
By the quality of your work. You can be a solo artist just starting out, but if you have a couple dozen logos that you have designed, a client will be able to see your style and your professionalism. You can redesign existing logos, or make up your own companies and brands to illustrate. You can show logos you design for competitions, such as DesignCrowd.com. Logos pretty much speak for themselves when it comes to "professional" looking...
By looking at their record of work, including asking for references and contacting them.
A "good" design agency will be able to reassure a client that they are professional by the attitude and approach they take to them and their work.
btw a company that calls itself a "logo design company" is probably not a fully developed agency – logo design is one aspect of corporate identity which is a much wider discipline – not every company will need a full identity but they do need to know that their logo is not the only element considered.
Portfolio, awards/other types of success, degree/other type of education or background in the field, customer service, duration of experience in the field, price (is it reasonable?), reviews...