How do you know if oil filter is dirty on my jaguar 1996 xj6?

Seriously? Is the oil dirty then the filter is dirty. Change it every 3-4k miles! On my 88 XJ6 I change it ever 4k miles since it does hold 8 quarts, on my 94 XJ12 I change it ever 6k miles as it holds 11 quarts. If you drive it like a race car in the dead of summer change it more often, if its really dirty before the miles are up then change it anyways. It's the most important thing to do for these old gals.