How can I verify how much refrigerant was put in my home AC unit?
You could refuse to pay the bill until they provide you with documentation of the pressure levels that prompted the addition of refrigerant and the amount of refrigerant required. They should have recorded this on the service sheet. Ask for a copy of their service sheet. Also, justification of their price of R-22 and why it is $125 per pound.

I believe HVAC contractors are required by the EPA to meticulously document all exchanges and uses of old refrigerants like R-22. 

They should have discussed all of this with you BEFORE they did any repairs or additions of refrigerant. Especially since the cost was going to be so high.

Good luck!