Went to NAPA yesterday with the idea of buying air filter monitors for each of my four diesels. At $24 each, and me with no income at the moment, I will instead take used plastic syringes, put springs inside of them, insert them into the suction sides of the filter/engine air intakes, and figure out over time where to make my marker pen marks as the filters clog up. If you want them to record the maximum vacuums, put an O-ring that snugly fits the inside of the syringe and will remain after the plunger returns on spring pressure.
Shhhhh, don't tell anyone my idea.