Ask a small business owner where their work comes from and you will get a confident answer: word of mouth, mostly, plus the website, and that directory we pay for. The confidence is genuine and the answer is frequently wrong, for a straightforward reason. It is assembled from memory, and memory over-weights the recent, the memorable and the pleasant. The customer who mentioned a friend's recommendation is remembered because the conversation was nice. The four who found the business through a search and said nothing about it are not remembered at all, because there was nothing to remember.
The consequence is that money and effort go to the wrong places. A directory subscription gets renewed for years because it is assumed to be working, and nobody has ever checked whether a single customer came from it. A social account absorbs several hours a week because it feels like the modern thing to do. Meanwhile the thing that is actually producing enquiries - which might be something the business does incidentally, or a single listing, or one referral partner - goes unrecognised and therefore uncultivated. The error is not that the business is investing badly; it is that it is investing blind, on impressions that were never designed to be evidence.
The measurement is one question, asked of every new enquiry: how did you hear about us. Written down in a column, consistently, for three months. That is the whole method, and its difficulty is entirely in the consistency - it has to be asked every time, including when busy, and it has to be recorded somewhere other than in somebody's head, or it produces the same impressionistic answer with extra steps. Three months of a small business's enquiries is usually enough for the pattern to be unmistakable, because the differences that matter are large rather than subtle.
What comes back tends to contain one genuine surprise, and it is often a source that produces few enquiries but very good ones, or a channel that produces a steady trickle nobody had noticed. There is one caution worth keeping: people answer this question imprecisely. Someone who was recommended by a friend and then searched for the business will often say they found it online, because that was the last step. So it is worth asking the second question occasionally - what made you get in touch with us specifically - which separates the route from the reason. The route tells you where to be; the reason tells you what to say when you are there.