I heard David C Baker talk about this.
Some people don't believe that they're an expert, when actually, the fact that you've been working at something for years and are quite good at it, makes you an expert.
One way to discover where your expertise lies, is to do a client matrix.
Take ten of your recent clients, projects that were successful, did OK or just weren't outright failures. Write these on a whiteboard down the side.
Then across the top, right out factors involved in that project. These can be all sorts of things - number of decision makers, have an internal marketing department, services or manufacturing company, head office is in Yorkshire, paid in instalments, used freelance graphic designers - any factors that could affect the project in any way.
Now you've got a grid - clients down the side, factors across the top. Now fill it in - tick the ones the clients with a head office in Yorkshire, fill in the number of decision makers involved.
And then look for patterns.
You'll probably spot something - our best projects were Yorkshire manufacturing companies that used freelancers for all marketing collateral.
And there's your expertise. Those companies are the ones that you can help best.
Take action: Draw up the grid and analyse the patterns