Answering this question reliably and consistently is just about the holy grail of web site marketing.
The more detail you know about how your favourite customer found you, the easier it becomes to find another one just like her (or him or it).
Why waste time, money and energy on anything but your most productive marketing?
You can easily see from your web stats how many visitors you get from Google for each keyword. But knowing that you had 15 visits from people searching for “lawn mowing wellington” does not tell you which of them are tyre kickers and which was the best customer you ever signed.
The good news is that you can include this information when somebody uses your enquiry form. So along with their contact details, you also see (for example) that they found you on google.co.nz using keywords “lawn mowing wellington”.
Now that’s the kind of intelligent marketing I want.