HVAC Repairs how to validate

[Gp G.]

Hi,

Recently I got ticket from PM app that HVAC is not working. PM HVAC technician when visited my property to attend attend ticket mentioned  my HVAC system  service valve core is bad and needs replacement of service valve core and add few lbs of refrigerant for about 700$. I sent my other HVAC technician who installed this new HVAC system about year ago for about 15k mentioned service valve core is good and refrigerant also good. But for tenant peace of mind he just added 1 lbs of refrigerant. System is working fine now and tenant also good now so far. So I feel cheated and lied by PM company HVAC contractor. Next time when I get HVAC repairs how to protected from cheating. What photos and what reading and what details I should ask HVAC contractor so that I will not be cheated.  Any codes or indicators or pressure readings does it show saying this is bad like that on HVAC inspection tools, readings. How remote investors handling these types of situations. I know some basics of HVAC but not expert of it though. What kind of intelligent questions i can ask when HVAC technician says xyz is bad cost thousands of dollars and weather is bad needs to be replaced right away and cannot guarantee next appointment as busy with hundreds of other appointments due to incremental weather etc.

 Please advise