JesseHVAC

This is close, and I'd grant that limit trips are associated with longer run times. But in principle continuous furnace run time shouldn't cause a high limit trip. The limit is mainly to protect the heat exchanger from damage. The primary cause is usually low/reduced airflow, although bad sensors are also common.

A high limit trip will generally cycle the furnace off for a time period. Somewhere between 3-5 off cycles will result in lockout and the furnace will stay off. The control for this is more likely to reside in the furnace board vs an aftermarket ui.

I would argue that hvac is uniquely bad

I'm the author of the article. The dual manometer suggestion below is excellent. I use external static pressure to calculate airflow using manufacturer tables quite frequently. It's fast and only slightly less accurate than using an expensive flow grid. I also use a manometer to measure pressure drops across filters and AC coils quite a bit.

I'd add a tool for measuring temperature, ideally a digital psychrometer, but even something that just measures dry bulb is useful for heating and bad AC failure.

Finally a multimeter, ideally with clamp on amp readings.