Checking for instability of an LNA is not as simple as it looks. Even an LNA is designed to amplify very small signals, most of the RF systems in use today, cannot ignore large input level behavior.
The assumed 50ohms inp/out loads, it must be stable for all input power values, with instability occurring only due to mismatch effects.
In some cases, the mismatch effects may also affect the behavior of the LNA at frequencies where a steady–state solution exist.