What would be the best way to inspect a roof top heat pump unit ?
Got to get up on the roof and remove service panels to check filters, condition/age of unit, etc, etc, etc
Hire an HVAC professional and watch him from your air conditioned vehicle.
We have many homes here in AZ with roof mounted heat pumps. Do not have many residential roof units with a filter at the unit, commercial yes, residential no.
With the tile roofs, I can see that the filter would be mounted in the return duct.
A Home Inspector inspects a Heat Pump by turning on the thermostat.
You can do that from inside the house.
Your question leads me to beleive that you don’t know what a Heat Pump is, nor are you certified to do this work.
Read the SOP.
Do not use made up rules-of-thumb.
If it heats/cools the house than report what you observed.
If you want to inspect HVAC, go back to HVAC school, get certified by the EPA, work in the field for about 4 years and then you may consider charging for this inspection.
Or, stick with Home Inspection.
Making any statement in your report that is not correct, is a liability.
Why not get you an HVAC guy who can actually test something, do the work with you?
Also, Fill out your profile so we know where you are. All things are not the same for all of us because of location.
Basically the same way you would a grade level heat pump unit. I agree with Dave that if you dont know how to do that you need some additional training.