(Like this one) are often audited by highly respected and trusted industry-owned, non-profit distributing organisations, like ABC Electronic
The audit involves careful studies of site traffic over a fixed period - say, a month - and the resulting statistics produce an ABC rating, which will be trusted and accepted by advertisers. The process can be expensive however, and your local football club probably wouldn't want that kind of financial commitment - or warrant it anyway.
I suggest that you start collating traffic figures from your server logs - usually available on request from your web hosting company. You can use the information you gain when you approach potential advertisers, but heed fourm member's warning - don't confuse 10,000 page requests with 10,000 unique visitors. Many people make this mistake, and it's important to understand what happens on the server.
When I visit a web site the server records my arrival (unique visitor) and it records a 'hit' each time I move from page to page. It doesn't take long for one unique visitor to clock up 10 hits, and you can see that 1000 visitors can easily result in 10,000 hits. A busy webmaster, checking and re-checking a site will easily record 100 hits a week, and if you add to that the number of hits as proud committee members check their club's site you will appreciate how easy it is to get a totally false impression of a site's true traffic figures.
Demonstrate that you really do have 10,000 unique visitors each month and you should have no trouble attracting advertising revenue from local companies.