Diesel Transit fuel filter service indicator?

How do I know if the fuel filter on my 04 2.4 transit van needs replacing. It says in the manual to look at the indicator on the top of the filter housing. Press the yellow button for 3secs, start the engine and let it run for 5secs at 4000rpm then check the service indicator again. If its in the green zone theres no need to change the filter but what should be in the green zone is it the fuel level in the glass bowl of the indicator ? How do I get the fuel filter out of its holder as it seems to be the original from when the van was new and is stuck fast in its fitting. Is there a way of checking the gearbox oil level on this van please.