This is a valuable question, but the replies are going awry of what draws me to the question. What if all you have to go on is a 'reply-to' email and their first and last name?
Not a complete list, but fishing for suggestions:
Context--they sent you a business related email, and appear roughly 'professional' to you, but you want to make sure.
Their message/website/page/example project. You be the judge.
Do they reply to their contact form?
Is their contact information available and current?
LinkedIn, github, any other free, populous site membership--what is their public face?
Being part of the social web is not an indicator of professionalism. However, you may learn about someone's professionalism based on what they have online if they participate. Keep in mind many good professionals have life outside of their work and don't have the time or interest in open source projects or giving free advice on sites like stack overflow.
>Being part of the social web is not an indicator of professionalism.
Not at all?
I'm not saying you need to be tweeting what you had for lunch or anything but having some presence on the social web is probably a good indicator in a broad swathe of fields. For example if I'm after a graphic designer and they're on LinkedIn|Facebook|Twitter or somesuch then it's an indicator that they're clued in with modern advertising, likely to have their finger more on the pulse, likely to be happy corresponding via the 'net, etc..
Not a complete list, but fishing for suggestions:
Context--they sent you a business related email, and appear roughly 'professional' to you, but you want to make sure.
Their message/website/page/example project. You be the judge.
Do they reply to their contact form?
Is their contact information available and current?
LinkedIn, github, any other free, populous site membership--what is their public face?