X-Gauge on 1KD-FTV engine: How to detect secondary fuel filter blockage?

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Those using a secondary filter in line filter the Toyota filter may realize that the blocked filter warning triggered by exceeding the suction threshold in the main filter sensor will not include any flow restriction contribution from a secondary filter. By the the time a low power situation due to low fuel delivery is noted the excess fuel flow for cooling purposes of the injector has long gone. This could be a worry, or is at least to me. I can't see any diagnostic value that indicates return fuel flow, or total flow or perhaps return fuel temperature. An indirect method may be to assume that typical fuel rail pressures can no longer be maintained with a fuel filter block. Does anybody else have another idea? To take off the return fuel line and measure what comes out is a bit messy and not all that practical and covers only an idle situation, ie low injector demand. To just change out filters following a general recommendation is at best "just general" advice, and could result in being actually overdue in adverse conditions. To change out early or by gut feel is just expensive and also messy while on the road. Second step would be whether that indicating sensor value can also be displayed conveniently with an X-Gauge command.