Filter-driers are rated based on a 1 psi pressure drop at its maximum flow capacity ratings, so that will give you an idea of what is “normal”. If the pressure drop exceeds that, it may be time to consider replacing it.
Consider this: If you measure any temperature drop between the inlet and outlet of the filter-drier, what would be the cause? Pressure drop.
Now consider this: If you have R22 liquid at 210 psig with 5°F of subcooling, what will the temperature of the liquid be if the pressure drops by 10 psi? by 15psi? by 20 psi?
For extra credit: If you measure the same R22 entering a filter-drier at 210 psig with 5°F LSC, what would the pressure drop need to be across the filter-drier if you measured a 10°F temperature difference, inlet to outlet?
Now Answer the Question: Should I replace the filter-drier? 😉