How do you know where your customers found you? Do you ask, maybe be use a form of tracking, have you thought about it?
How ever you promote yourself it essential you know how effective any aspect of your marketing is working. But I'm sure I don't need to tell you that.
What might help are a couple of inexpensive ideas.
Do you have a telephone number prominent on your website? Is that your normal office number because if it is you won't know they found you via the website. Setting up a second line, especially with a VOIP service like Vonage isn't expensive. So when somebody calls that number you'll know where they found you.
Do you have an email link on your website? Well the first thing I'd suggest is get rid of it. It is the ultimate spam magnet. Replace it with a form protected by Recaptcha, this goes a long way to preventing the junk coming through your form. We use it here. The contact form looks professional and ensures you get the essential information to be able to follow up on the enquiry. It also can send a confirmation email to the sender so they know you got the message. The message and the details are held on the website Database which you can download as a spreadsheet file if you want to create a mailing list. Though you should really be registered with ICO (Information Commissioners Office).
What do we do if we manage your website? We always add tracking tools like Google Analytics and Webmasters tools. This means we know who is finding your site (though not individuals). Which part of the world the are looking from, what key words phrases they may have used and a great deal more.