How can I check my alt , 200 amp

[BigTruckDriver]

I bought a 200 amp alt and its not working right. In gear at idle the voltage drops to 12.3 volts and the lights go dim. When the rpms are up it charges at 13.6 volts. I contacted the seller and he said the belt was slipping. The belt is brand new and there is no squealing. They then sent me a smaller pulley saying that was the problem. I changed the pulley and the alt seemed to jump up in voltage at idle but in gear it fell down again to the low 12's. After its warmed up it stays low and wont come up above 13 volts even at idle out of gear. Battery (1000 cranking amps) and belt are about 2 months old and I have upgraded my power and ground cables to 4 ga. wire.

The company has been answering all my e-mails and have been helpful but still insist the problem is with my vehicle. I pulled the alt and sent it back so they could check it over ,since I was not having any major problems with the stock alt. I was just trying to upgrade.

How can I check the alt to see if i am getting what I paid for?