If you want to know whether your opamp is stable or not you should simulate the open loop ac analysis with the actual loads and input impedances such opamp will be driving when closing the loop.
The stability analysis should always be performed on the open loop and not in the closed loop. Therefore stability criteria apply for open loops only, you cannot get PM and GM information if the loop is already closed.


Tosei

To "complete the confusion" here is my summary:

Both, Tosei and Ken, are right - however, perhaps their contributions sound contradictionary, but they aren´t !  
Phase as well as gain margins are defined only for the open loop system (of course, otherwise no input/output can be defined for loop gain). However, it is best and sometimes necessary to measure/simulate it in a closed loop configuration. For this purpose,Middlebrook has recommended a specific simulation topologie. For simple cases (when there is no severe load problem) you can simply place a series voltage source within the feedback path and let calculate Vout/Vin.