If you are using cadence, use the K-Factor simulation to ensure stability, for more information about this simulation check spectre manual
I you are using any other simulator, make S-Param simulation, and then you can get the K factor and the Delta factor from them,I dont remember the expressions but you can find them in any PA text, K must be greater than one and delta smaller than one...
PA's generally have to types of instability, low freq due to supply and ground termination problems and high frequency due to improper input and output matching.