While your filter is dort of transparant and you can see it discolor over time, you really cannot use that as a gauge. This is why filters are changed on a schedule. The fuel filter is the cheapest of the filters, so, as it is called for in the maintenance schedule, simply change it.

Same with an air filter, you could pull it out and look at it, but, there really is not an easy way to measure how much air is passing and how clogged it may be.
Change it on the suggested schedule which can be found in your owner's manual.

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